Method and apparatus for integrating wireless communication and asset location

ABSTRACT

A method and apparatus to identify tags in a wireless tag identification system. Operation of the wireless tag identification system may be controlled to minimize interference or otherwise improve interoperation with a wireless communication system, such as a wireless local area network (WLAN). For example, the timing and/or power of wireless signals produced by the wireless tag identification system can be controlled to provide sufficient space to the wireless communication system to operate properly. The systems may be tightly integrated, e.g., so that tags may be identified by responding to wireless communications traffic as opposed to special purpose tag search signals. The wireless communications traffic may be modified to enhance tag location resolution.

RELATED APPLICATIONS

[0001] This application is related to and claims the benefit under 35U.S.C. §119(e) of U.S. provisional applications No. 60/160,460, filedOct. 21, 1999, No. 60/181,848, filed Feb. 11, 2000, No. 60/183,193,filed Feb. 17, 2000, No. 60/191,030, filed Mar. 21, 2000, No.60/216,242, filed Jul. 6, 2000, No. 60/239,593, filed Oct. 11, 2000, and601247,080, filed Nov. 10, 2000, and the benefit under 35 U.S.C. §120 ofU.S. non-provisional applications Ser. No. 09/517,606, filed Mar. 2,2000, and Ser. No. 09/694,767, filed Oct. 23, 2000, which are all herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] This invention relates to location identification of mobile andstationary assets and communicating using a wireless network.

BACKGROUND OF THE INVENTION

[0003] Wireless communication has become increasingly popular, and theadoption of Wireless Local Area Network (WLAN) technology seems likelyto follow that of conventional Local Area Network (LAN) technology inthe 1980s. LANs were typically installed to support the sharing ofcentralized resources, such as printers, backup equipment, and centraldisks. Over time, network services were extended to include email,workgroup software, and the Internet. By 1995, it became almostunimaginable to have a modern office without a LAN. A WLAN is a naturalextension of conventional LAN technology, and is usually justified andinstalled based on a single application, such as support for arelatively small population of mobile devices, or support for cordlesstelephony. Once the infrastructure is in place, coverage and additionaldevices may be added to the WLAN incrementally.

[0004] Monitoring and tracking the location of assets, such aspersonnel, inventory, vehicles, and so on, in a facility can beimportant, e.g., to ensure the safety, proper allocation, or appropriateuse of the assets. One class of several different solutions that hasbeen used to track assets is a Local Positioning System (LPS). Oneparticular type of communicate over radio frequency bands with tagsattached to assets that are to be tracked by the system. The tags andthe antenna transceivers communicate using radio frequency communicationbands, and the information gathered from the tags is used by the systemto generate useful data about the tagged assets, such as the location ofthe assets. Determining a location of the tags as used herein mayprovide a general area in which a tag is located (e.g., a room or otherzone indication), a precise position of the tag (e.g., 2 or 3dimensional coordinates of the tag relative to a reference point), adirection in which the tag is located relative to a reference directionor point, or any other suitable indication of the tag location.

[0005] Equipment for and installation of a dedicated wireless tagidentification system infrastructure can in some cases be expensive, andwhen the total number of assets to be tracked is small, the cost perasset may be prohibitively high. In addition, a facility wishing to addan asset tracking resource to its operations may already have one ormore wireless communication systems installed and operating within thefacility. Thus, adding an additional communication system to thefacility may be cumbersome, interfere with other communications, and/orrequire unwanted additional expense.

SUMMARY OF THE INVENTION

[0006] In an illustrative embodiment in accordance with one aspect ofthe invention, a wireless communication system and a wireless tagidentification system are configured to operate in a common environmentwhile minimizing disruption of both wireless communication system andasset location activities. The wireless communication system may be anytype of communications network that carries communications information,such as audio, video or data information, e.g., a WLAN, mobile telephonenetwork, etc.

[0007] In one embodiment in accordance with one aspect of the invention,a method for determining a location for assets includes providing aplurality of assets, and producing a wireless communication signalinvolving a mobile device. The wireless communication signal representscommunications audio, video or data information. A frequency shiftingtransponder is used in conjunction with the wireless communicationsignal to locate at least one of the assets.

[0008] In another embodiment, a method for controlling operations of awireless communication system and a wireless tag identification systemhaving at least partially overlapping coverage areas includes providinga wireless communication system having at least two wirelesscommunication devices adapted to communicate by wireless signals, andproviding a wireless tag identification system adapted to communicate bywireless signals with at least one tag associated with an asset. Thewireless signals produced by the wireless tag identification system arecontrolled to minimize interference of the wireless signals withwireless communication of the wireless communication system.

[0009] In another embodiment, a method for identifying tags includesproviding at least one tag adapted to transmit a wireless signal, andproviding a wireless tag identification system adapted to receive awireless signal from the at least one tag and determine a location forthe tag. A first technique is used to determine the likelihood that thetag is within acceptable communication range, and a second technique isused to collect data from the tag if the tag is determined likely to bewithin an acceptable communication range.

[0010] In another embodiment, a method for identifying assets includesproviding at least one tag adapted to transmit a wireless signal, andproviding a wireless tag identification system adapted to receive awireless signal from the at least one tag and determine a location forthe tag. A wireless signal is received from the tag including a tagdatagram in which an error checking code portion of the tag datagram istransmitted at the start of the tag datagram.

[0011] In another embodiment, a method for identifying assets includesproviding a plurality of tags adapted to transmit wireless signalsincluding different length header portions, and providing a wireless tagidentification system adapted to receive a wireless signal from the tagsand determine a location for the tags.

[0012] In another embodiment, a method for communicating withcommunication devices in a wireless communication system and tagsassociated with assets in a wireless tag identification system includessending and receiving wireless signals to and from communication devicesin the wireless communication system, and receiving a second wirelesssignal sent from a tag in response to a first wireless signal. The firstwireless signal is sent from at least one communication device in thewireless communication system, and the first wireless signal is notaddressed to the tag. The second wireless signal is used to estimate thelocation of an asset.

[0013] In one illustrative embodiment in accordance with one aspect ofthe invention, a system for determining a location for assets includesmeans for producing a wireless communication signal in a wirelesscommunication system including a mobile communication device, thewireless communication signal representing communications audio, videoor data information, and asset locating means, including at least onefrequency shifting transponder, for using the wireless communicationsignal to locate at least one of the assets.

[0014] In another embodiment, a wireless tag identification systemincludes a plurality of tags each associated with an asset, and at leastone tag sensor adapted to communicate by wireless signals with at leastone tag. The at least one tag sensor has a coverage area within whichthe tag sensor can communicate with tags. Means for controlling wirelesssignals produced by the at least one tag sensor minimize interference ofthe wireless signals with wireless communication of a wirelesscommunication system taking place within the coverage area of the atleast one tag sensor.

[0015] In another embodiment, a wireless tag identification system foridentifying tags includes at least one tag adapted to produce a wirelesssignal, and at least one tag sensor that receives a wireless signal fromthe at least one tag. Also included are means for determining anidentity of the tag based on the wireless signal received from the tag,and means for controlling how wireless signals are generated by the tagsensor. The means for controlling uses a first technique to determinethe likelihood that the tag is within acceptable communication range,and uses a second technique to collect data from the tag if the tag isdetermined likely to be within an acceptable communication range.

[0016] In another embodiment, a system for identifying assets includesat least one tag adapted to produce a wireless signal including a tagdatagram in which an error checking code portion of the tag datagram istransmitted at the start of the tag datagram. Also included are at leastone tag sensor adapted to receive a wireless signal from the at leastone tag, and means for determining a location for the tag based on thereceived wireless signal.

[0017] In another embodiment, a system for identifying assets includes aplurality of tags adapted to produce wireless signals includingdifferent length header portions, and at least one tag sensor adapted toreceive a wireless signal from the at least one tag. Also includes aremeans for determining a location for the tag based on the receivedwireless signal.

[0018] In another embodiment, an integrated system for communicatingwith communication devices in a wireless communication system and tagsassociated with assets in a wireless tag identification system includesmeans for sending and receiving wireless signals to and fromcommunication devices in the wireless communication system, and meansfor receiving a second wireless signal sent from a tag in response to afirst wireless signal, said first wireless signal being sent from atleast one communication device in the wireless communication system, andsaid first wireless signal not being addressed to the tag. Also includedare means for using the second wireless signal to estimate the locationof an asset.

[0019] The following description and the appended drawings set forth indetail certain explicatory embodiments of the invention. However, theseembodiments are indicative of but a few of the numerous ways in whichvarious aspects of the invention may be employed. Other objects,advantages, and novel features of the invention will become evident fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Illustrative embodiments of the invention are described withreference to the following drawings, in which reference numbers indicatesimilar elements, and wherein:

[0021]FIG. 1 is a schematic block diagram of an illustrative wirelesstag identification system in one embodiment in accordance with an aspectof the invention;

[0022]FIG. 2 is a schematic block diagram of an illustrative embodimentincluding a wireless tag identification system and two wirelesscommunication systems;

[0023]FIG. 3 is a schematic diagram of an illustrative search procedurefor identifying a tag;

[0024]FIG. 4 illustrates how tag transmission collisions may occur andhow tag datagram adjustment may affect collisions;

[0025]FIG. 5 is an illustrative flow diagram for duty cycle control in awireless tag identification system;

[0026]FIG. 6 is a schematic block diagram of a tag sensor that mayfunction as an access point for a WLAN in an illustrative embodiment;

[0027]FIG. 7 illustrates how a signal from one interrogator may bereceived at another interrogator as well as be transponded by a tag;

[0028]FIG. 8 is a schematic block diagram of an 802.11b access point inan illustrative embodiment;

[0029]FIG. 9 illustrates two 802.11b data frames having different lengthheader portions;

[0030]FIG. 10 illustrates an illustrative tag datagram;

[0031]FIG. 11 shows several different tag transmission and receptiontimings in an illustrative embodiment;

[0032]FIG. 12 illustrates how a tag transponded DQPSK signal may bedecoded;

[0033]FIG. 13 is a schematic block diagram of an LPS signal processor inan illustrative embodiment; and

[0034]FIG. 14 is a flow diagram of operation for an integrated tagidentification/wireless communication system.

DETAILED DESCRIPTION

[0035] As discussed briefly above, a Local Positioning System (LPS) isone type of wireless tag identification system that is designed to trackthe locations of tags as they move through a facility. In somecircumstances, users may wish to employ an LPS alongside a wirelesscommunication system, such as an 802.11-based WLAN. In such cases, itmay be necessary to coordinate LPS and WLAN traffic in such a way thattags may be continuously tracked without interfering with WLANcommunication. Thus, it may be desirable to design a system that wouldallow LPS and WLAN traffic to coexist without interference, or minimalinterference. In addition, users of wireless communication systems mayprefer not to install multiple infrastructures. Thus, in order tocoordinate LPS and WLAN traffic while decreasing the need to install anadditional infrastructure, it may be desirable to design a system withintegrated LPS and WLAN capabilities.

[0036] Today, LPSs and WLANs are promoted, justified, and installedseparately. Sometimes a WLAN is installed first, with an LPS addedlater. Other times the order is reversed. Thus, in many cases, both LPSand WLAN capabilities are of interest to end users. Since an LPS andWLAN may be installed in the same physical space, or in partiallyoverlapping spaces, there may be an advantage to providing a WLAN andLPS as a single, properly functioning package. Products that areintegrated in this fashion may have a distinct commercial advantage overstand-alone products from at least three important perspectives:

[0037] 1. End users who require both LPS and WLAN solutions will tend tofavor solutions with a clear upgrade path. Thus, emerging LPS productsthat do not harmoniously share the radio channel with WLANs may be at acommercial disadvantage, particularly in sites where such WLANs arealready in place.

[0038] 2. With the emergence of the 802.11 WLAN standard, WLANs arebecoming commodities. The hardware is increasingly being promoted withan emphasis on system-level features. Wireless systems that seamlesslysupport LPS with the same

[0039] 1 infrastructure may have an important competitive advantage.

[0040] 3. A smooth upgrade path from WLAN to LPS and vice versa createsa follow-on sales opportunity once the initial infrastructure is inplace.

[0041] Below, the inventors describe a variety of approaches to achieveinteroperability and/or integration between a WLAN and LPS. Although aWLAN and LPS are used in the illustrative embodiments, the invention isnot limited to these system as the various aspects of the invention maybe used in other environments. Thus, although illustrative embodimentsare described with reference to the PinPoint LPS or derivative designs,the various aspects of the invention may be used with other wireless tagidentification systems, such as those that use radio frequency signals,Global Positioning System (GPS), E-911, Doppler shift, infrared signals,ultrasonic signals, or any other signal or technique, or combination ofsignals and techniques. For example, the techniques described below,especially with regard to duty cycling the interrogator and the use of aCCA, may be advantageously applied in any Radio Frequency Identification(RFID) interrogator that may share the radio channel with a wirelesscommunication system. (LPS tags are usually considered ahigh-performance type of RFID tag.) These same techniques may be appliedto other types of RFID and other wireless tag identification systems,and no limitation is intended. Similarly, several of these techniquesmay also be advantageously applied to prevent nearby interrogators—ofthe same type and of different types—from interfering with each other.

[0042] A wireless communication system operating in accordance with thewidely used 802.11 WLAN protocol is used herein as a primary example ofa WLAN. An understanding of the 802.11 protocol (Direct Sequence SpreadSpectrum, or DSSS), which is publicly available from IEEE (Institute ofElectrical and Electronics Engineers), is assumed. Thus, a detaileddescription of the operation and components of a WLAN is not providedherein. As with the LPS, various aspects of the invention are notlimited to use with a WLAN, much less an 802.11 compliant WLAN. Instead,any suitable wireless communication system may be used, such as thosethat use radio frequency signals, infrared or other electromagneticsignals, ultrasonic signals, or any other suitable signal type orcombination of signal types. The wireless communication system mayinclude all mobile communication devices, all fixed communicationdevices, or any suitable combination of the two, and may use anysuitable protocol or other communication format. A typical wirelesscommunication system may have fixed communication devices, such asaccess points in a WLAN, that communicate with mobile communicationdevices, such as mobile telephones, personal digital assistants, etc.

[0043] In the description below, specifications of both a 802.11 LAN andone type of LPS are discussed to identify some of the considerations forcoordinating LPS and WLAN traffic, and methods for coordinating channelusage between an 802.11 WLAN and an LPS are described, e.g., a systemwith integrated WLAN and LPS capabilities in a single infrastructure.Although the illustrative embodiments described below focus mainly onthe tight integration of LPS with 802.11 WLANs, the same or similartechniques, systems and methods described herein may be used with otherwireless communication system technology standards, such as DECT andBluetooth. Thus, the same general concepts apply to other products, andthe techniques are not intended to be restricted to 802.11 technology.Quite to the contrary: the basic scheme may be flexible enough tointeroperate with all WLANs or other wireless communication systems ofconcern to end users, and only a few of the techniques described hereinare specific to 802.11.

[0044] Wireless Tag Identification System Infrastructure

[0045] As shown in FIG. 1, an LPS, such as PinPoint's LPS, typicallyincludes a set of interrogator antenna modules 5 arranged in anenvironment that can communicate over radio frequency bands with tags 2attached to assets (not shown) that are to be tracked by the system. Thetags 2 and the antenna modules 5 communicate using radio frequencycommunication bands, and the information gathered from the tags 2 isprovided to an interrogator 6 and a host computer 7 and used to generateuseful data about the tagged assets, such as the location of the assets.The term tag sensor is used herein to refer to any device that iscapable of detecting the presence of a tag, and may include an antennamodule, interrogator, tag reader or any other suitable device. Detailsregarding the operation of such an LPS have been previously disclosed,for example, in the applications incorporated above. Thus, a detaileddescription of the system operation and components not directly relatedto aspects of the invention is not provided herein. As discussed below,an LPS may be installed in an environment shared by one or more WLANs,and the LPS may be configured to incorporate many of the various aspectsof the invention.

[0046] In one illustrative embodiment, one or more WLANs and an LPS maybe installed side by side, each with its own infrastructure, as shown inFIG. 2. In this illustrative embodiment, an LPS is operated in anenvironment in which the LPS coverage area, i.e., the area within whichthe LPS may communicate with tags 2 and determine tag location, overlapsat least partially with a coverage area of the two WLANs controlled byWLAN controllers 40 a and 40 b, respectively. Although in this example,two WLANs and a single LPS are shown, any suitable number of WLANsand/or LPSs (or other wireless communication systems and wireless tagidentification systems) may be used. In addition, the LPS is shown asincluding three interrogators 6 that each communicate with four antennamodules 5, but it should be understood that the LPS may include anysuitable number of interrogators 6 and/or antenna modules 5 and anysuitable number of antenna modules 5 may communicate with eachinterrogator 6. In addition, other wireless tag identification systemsmay not use the interrogator/antenna module structure of the LPS and/ormay use mobile tag sensors (not shown) to communicate with tags. Inshort, any suitable arrangement for a wireless tag identification systemmay be used as the invention is not limited in this regard.

[0047] The same is true for the WLANs. In this illustrative embodiment,each WLAN includes three access points 3 that communicate with a WLANcontroller 40, but any number of access points 3, controllers 40 and anyother components may be used in the wireless communication system. Aswith the LPS controller 7, the WLAN controller 40 may include anysuitable components to perform desired functions, such as one or anetwork of general-purpose computers and/or special purpose devices, anysuitable software modules, hardware, firmware, or other componentsnecessary to perform desired input/output, analysis, reporting,communication or other functions. For example, 802.11 access points andPinPoint LPS interrogators are commonly implemented as devices that plugdirectly into an Ethernet LAN, and devices on the LAN may access thesedevices directly. Controller functions may be implemented as softwareservices running on servers on the LAN and/or may run on designatedaccess points or interrogators on the LAN. The access points 3 mayoperate using the 802.11 standard or any other suitable protocol forcommunicating wirelessly with devices 4 in a network. In thisillustrative embodiment, the access points 3 may include a directsequence 802.11 standard transmitter/receiver that transmits andreceives communication signals with respect to communication devices 4in the WLAN.

[0048] Under certain conditions, an LPS operating in the 2.4 GHz bandmay interfere with the operation of WLAN products also operating in the2.4 GHz band. Because both is WLANs and LPSs operating in the 2.4 GHzband are being installed more and more frequently, it may be desirableto lessen the possibility and/or the effect of interference between LPSsand WLANs. Properly installed systems will usually not have across-interference problem. For example, in many casescross-interference can be essentially eliminated by placing LPS and WLANantennas in different positions and/or orientations (e.g., LPS antennamodules 5 may emit radiation using directional antennas that aredirected away from access points 3), at compatible power levels, and atdifferent center frequencies. However, despite these techniques, undercertain conditions, LPS interrogators 6 operating in the 2.4 GHz ISMband may degrade the performance of WLANs also operating in the 2.4 GHzband, or vice versa.

[0049] In accordance with one aspect of the invention, the timing atwhich the wireless tag identification system is permitted to transmitwireless signals, e.g., to communicate with tags 2 and determine theirlocation, may be adjusted to minimize interference with wirelesscommunication of the wireless communication system, such as a WLAN.

[0050] Duly Cycle Control

[0051] One approach for limiting interference between an LPS and theWLAN is to adjust the duty cycle at which one or more antennas 5 and/orinterrogators 6 in the LPS operate. The term duty cycle as used hereinrefers to any suitable timing control of when any portion of thewireless tag identification system is permitted to transmit wirelesssignals. Thus, a duty cycle may be a regular, periodic timing at whichthe wireless tag identification system is permitted to transmit, or maybe an irregular, random or otherwise non-periodic timing. For example,duty cycle control software, hardware and/or firmware may be implementedin the LPS controller 7 and/or the interrogator 6 and/or any othersuitable processing apparatus that communicates within or with the LPS.For example, an interrogator 6 might be “on”, e.g., permitted totransmit wireless signals, for 250 milliseconds every second, and “off”for other portions of the time. One implementation of duty cycle controlmay be based on three parameters: (a) Fixed On duration (for example 250milliseconds); (b) Fixed Off duration (for example 500 milliseconds);and (c) Randomized Off duration (for example 500 milliseconds). In thisexample, the interrogator 6 may turn on for a period of 250milliseconds, and then turn off for a period of 500 milliseconds. Beforeturning on again, the interrogator 6 may generate a random number from 0to 500, and wait the random number of milliseconds before turning onagain. The result may be an average duty cycle of 0.25 seconds on and0.75 seconds off. The randomized component may prevent the interrogatorcycle from occurring in synchronization with other periodic processes,e.g., such as those occurring in the WLAN wireless communications. (Inthe case of the PinPoint LPS, tags 2 themselves transmit on a periodicschedule. Other processes, such as WLAN “heartbeats,” might be onsimilar periodic schedules.)

[0052] For LPSs operating at a relatively high power, e.g., above 100milliwatts, such duty cycle control may enable the system to operatelegally, if regulations allow emitted power to be averaged. For example,during an On portion of the duty cycle, the LPS may emit a power levelthat is too high to be in accordance with regulations if the power levelwas sustained. However, if the on portion of the duty cycle issufficiently short compared to the Off portion of the duty cycle and theregulations permit power averaging, the average power emitted by the LPSover the on and off periods may be within an acceptable range.Alternately, the LPS may adjust the power of signals emitted by one ormore interrogators and/or antennas to minimize interference with theWLAN. Thus, power control may be performed dynamically, e.g., inresponse to WLAN activity, and/or on an antenna or interrogator basis.

[0053] These and other operational parameters of the WLAN and/or the LPSmay be set to achieve the desired performance tradeoff of the WLAN vs.LPS. For example, an installer of the WLAN and/or LPS might refer to amanual to determine optimal parameters for WLAN/LPS interoperation.Alternately, for example, the configuration software for the LPS mightallow a user to specify a WLAN product with which the LPS is tointeroperate and the level of interferece that is acceptable; based onthis specification, the LPS configuration software may set the detailedparameters using a lookup table or other technique. For example, inaddition to setting duty cycle or power control and other parameters, incases where an interrogator 6 uses a WLAN for communication back to theLPS controller 7, WLAN data transmissions for the interrogator 6 maytake place during the interrogator Off periods.

[0054] Another potentially important optimization that may be used is tosynchronize the duty cycle or other operation timing among nearbyinterrogators 6 and/or antennas 5. For example, if each interrogator 6has an independent duty cycle, an Off time from one interrogator mayoccur during the On time of another interrogator 6. Thus, the periods ofnon-interference may be accordingly reduced for WLAN access points 3 anddevices 4 in range of both interrogators 6. Synchronizing the dutycycles among the nearby interrogators 6 may mitigate this effect. Incases like the PinPoint LPS, where the interrogators 6 are on a localarea network, one interrogator (or the host, i.e., the LPS controller 7)may be set up as the Master with the other interrogators 6 as Slaveswhen it is time to turn on, the Master may send a Turn On commandincluding duration to the Slave interrogators 6. Due to packettransmission time over the LPS network and operating system delays, itmay take a few milliseconds for this command to actually be received atthe interrogators 6; therefore, for best effectiveness, the On durationmay be a significant multiple (say 10 times or more) of such latencytime. To some extent, for critical applications, such latency may becalibrated at installation and adjustments may be made accordingly,e.g., the turn on command may be sent in advance of a desired turn ontime for the interrogators to account for transmission delay.

[0055] Alternately, interrogator synchronization may be accomplished bysending a schedule from the Master to the Slaves. For example, theMaster may send a schedule for the next ten On/Off cycles to the Slaves.If the network is known to have approximately a seven-millisecond delaybetween a particular Master and a particular Slave, the Master can sendthe message seven milliseconds early. Alternatively, if the network hasa reliable time synchronization facility, the duty cycle timing may bebased on the system clock, not the time of receipt. The On duration forthe interrogators 6 and/or antennas 5 may vary according to the responsefrom tags 2 in the coverage area for the interrogator/antenna. Thisnotion is especially useful for LPSs with passive (i.e., no battery)modulated backscatter tags. If there are no tags in range, this fact maybe apparent from the lack of modulated backscatter energy received bythe interrogator 6 or antenna 5. (Even in the event of two tagsoperating simultaneously, such energy may be present.) Thus, the Onduration may be set to the minimum required to see a single tag. Once asingle tag is seen, the interrogator or antenna may be left on for aslong as it takes to see all tags with the desired reliability.

[0056] An LPS may perform two basic functions: detecting newly arrivingtags, and verifying that tags are still in range alter they have beenpreviously detected. Some optimizations in accordance with one or moreaspects of the invention described below, may be used to lock onto newtags efficiently. In the case of PinPoint LPS, if the interrogator haspreviously seen a tag, it may forecast the tag's transmission time withreasonable accuracy. Forecasting may be possible if the randomized Offduration for the tag is based on a formula that incorporates the tag'sserial number or some other known value. Since the interrogator, LPScontroller or other portions of the wireless tag identification systemmay know the tag's serial number, once it has received a transmissionfrom the tag, the tag's serial number can be used to forecast the tag'snext transmission time. A randomization algorithm that may be used tocontrol tag transmission timing is described in a subsequent section.

[0057] At the time of a forecasted tag transmission, one of thefollowing things may happen: (1) The tag is read; (2) No in-band RFenergy is received; (3) In-band RF energy is received at the expectedtime, but is interpreted as a corrupted datagram or other informationindicating that at least one tag was transmitting at the expected time;(4) The interrogator's duty cycle was Off. Case 1 is unambiguous—the tagis determined to be present. Case 2 strongly suggests that the tag hasmoved out of range or has been obstructed. Case 3 is ambiguous, butsuggests that the tag is probably still present. Case 4 provides nouseful information. In Case 4, and as required in Case 3, software inthe LPS controller 7 or other host computer may modify the duty cycle ofan interrogator, antenna or other portion of the wireless tagidentification system to look for a particular tag that has not beenseen for a long time. For example, suppose the presence of every tagneeds to be positively confirmed once per minute. Also, suppose that onewould statistically expect 99.9% of tags to be seen once per minute.Therefore, if a particular tag has not been seen for a minute, the dutycycle or other operation timing can be adjusted to be On when that“missing” tag is expected to transmit. In this simple example, it willonly be necessary to adjust the duty cycle once per minute for just 0.1%of the tags.

[0058] As mentioned above, the duty cycle or other operation timing maybe set to vary by antenna 5. Certain areas, such as portals, hallways,doorways, etc., may be able to tolerate higher WLAN interference inexchange for higher LPS system performance. In one embodiment, eachantenna may have a user-settable priority level, such as 1=High (e.g.,for a portal); 2=Medium (e.g., for a loading dock area); 3=Low (e.g.,for a permanent or semi-permanent storage area); 4=Very Low (e.g., for ahigh WLAN traffic area in which minimum interference is desired). Also,the duty cycle or other operation timing may be adjusted based on a timeof day, day of week, etc. For example, during certain times of day, suchas after hours in an office, WLAN activity may decrease, and thus usersmay be able to tolerate higher WLAN interference in exchange for higherLPS system performance. (After normal work hours in an office, theretypically are few individuals in the office and there is a low level ofWLAN activity. Setting a higher duty cycle, e.g., that allows a longeror higher percentage time for LPS wireless transmission, during thosehours may allow the LPS to more readily detect unauthorized movement inthe office, such as equipment theft.) The Master (described earlier) mayinclude a priority level in its On commands. For antennas with a LowPriority Level, techniques may be used to minimize interference withWLAN activity, e.g., power levels for the antennas may be attenuatedand/or directional antennas may be employed to limit the extent of RFemissions. Power levels may be attenuated on a per-antenna basis, thatis, each antenna may have a different attenuation setting, e.g.,depending on the proximity of the antenna to a WLAN traffic center (anarea in which relatively higher levels of wireless communications takeplace). Adjustment to the duty cycle or other operation timing may bemade automatically by the LPS controller 7 or other portion of thewireless tag identification system based on detected WLAN traffic in anarea near one or more antennas. Detected WLAN traffic levels may bestored and used to develop a WLAN activity history that is then used todetermine optimized wireless tag identification system operatingparameters, such as duty cycle control, antenna power levels, etc.

[0059] In another illustrative embodiment, a variable duty cycle for theLPS wireless operations such as that described above may be combinedwith special embedded tag software to extend tag battery life. Forexample, on a relatively frequent basis, such as every two seconds, atag may be programmed to wake up in two stages. During the first stage,the tag may turn on its receiver circuitry and monitor the receivedwireless signal strength at 2.4 GHz. If the tag detects a receivedsignal level above a fixed threshold, and/or if variations in thereceived signal power resemble the pattern typically emitted by an LPSinterrogator in search mode, the tag may turn on its transmitter after ashort delay. Such delay may vary with the details of interrogator andtag operation. If no received signal is detected, the tag transmitter isnot enabled. Since the tag uses much less power in receive mode, thisapproach may save battery power. However, the tag may not be completelyaccurate in determining whether an antenna is searching for a tag, asother 2.4 GHz emitters might be mistaken for an interrogator signal,and/or the interrogator/antenna may be too far away to trigger thethreshold. Therefore, to ensure the tag is seen occasionally, the tagmay transmit periodically whether or not incoming 2.4 GHz energy isdetected. Similarly, a tag may be programmed to decrease its own rate ifit sees incoming energy for a consecutive number of transmissions, or ifan acknowledge (ACK) command is sent to the tag (e.g., via amplitudemodulation) in response to its transmission.

[0060] The aforementioned techniques as with other aspects of theinvention described herein, including varying duty cycles and powerlevels, may be advantageously applied in any suitable combination oralone. The wireless tag identification system may use a graphical userinterface (GUI) to allow a user to set/select multiple operationalparameters for the system. For example, an installer might refer to amanual to look up optimal parameters for interoperation with variousWLAN products. The installer may then provide values for theseparameters using the GUI. In another illustrative embodiment, thewireless tag identification system configuration software might allow auser to specify a WLAN product with which the wireless tagidentification system is to operate, and the level of interference thatis acceptable. Based on this selection, the system may set the detailedparameters from a lookup table or other information source. The wirelesstag identification system may also be made “intelligent” so that it canuse information, such as detected WLAN traffic near one or moreantennas, e.g., using a CCA-type function described below, and adjustoperating parameters of the system, such as a duty cycle for one or moretag sensors, to optimize tag location performance while minimizinginterference with the WLA wireless communications. For example, when thewireless tag identification system is initially set up, an antenna maybe believed to be located in a high WLAN traffic area and may be given arelatively low priority level. However, over time, the wireless tagidentification system may determine that WLAN activity is not highenough in the vicinity of the antenna (at least at certain times of theday or week) to justify the low priority. In response, the prioritylevel of the antenna may be increased, along with the percentage timethat the antenna is permitted to transmit wireless signals.

[0061] Randomization Algorithm for Tags

[0062] The algorithm used to control a sleep time for a tag may takeseveral considerations into account. For example, the algorithm mayprevent pairs of tags from re-colliding repeatedly, and enable theinterrogator or associated software to forecast tag transmission times.The algorithm may make optimal use of the limited capabilities ofinexpensive microprocessors suitable for use in a tag, particularly withregard to power management, code space, and memory space.

[0063] In one illustrative embodiment in accordance with an aspect ofthe invention, a tag sleep algorithm endeavors to make optimal use ofcapabilities available in the PIC family of 8-bit microprocessorsavailable from Microchip. PIC processors are illustrative of featuresavailable in inexpensive processors, and firmware running on PICprocessors may also be implemented in custom devices (such as an ASIC)for further cost-reduction. PIC processors support two main timingtechniques. A programmer may place the processor in a timed loop, with aresulting accuracy defined by the processor's crystal oscillator. Whilefairly high in precision, this approach also uses relatively high power.For lower accuracy at lower power, the PIC provides a watchdog timerbased on an RC circuit, which allows the programmer to place theprocessor in a low-power state for a prescribed period of time. When theprocessor is programmed, the user may select one of the following fixedvalues for the watchdog timer: 18, 36, 72, 144, 288, 576, 1152, or 2304milliseconds. The tag's low-power sleep cycle may be a multiple of thispre-selected value.

[0064] Suppose a sleep time is desired that varies pseudorandomly in therange of 5±0.5 seconds. If a watchdog timer period of 72 milliseconds isselected, this creates approximately 5000/72=69 slots over a five secondperiod. A sleep time in the range of 5+0.5 seconds may be achieved byrandomly selecting a set of watchdog intervals in the range of 63 to 76,as shown in the following pseudocode: Set watchdog timer at 72milliseconds; While (true) { Select a pseudorandom Number from 63 to 76;For i = 1 to Number {sleep for 72 milliseconds}; Transmit tag datagram;}

[0065] This algorithm provides better than expected performance due tosmall hardware differences between tags. In some cases, the watchdogtimer may be accurate only within a range of about 30%. In practice,this degree of variation has not been observed, but significantdifferences from one component to another and at different temperatureshas been observed. Despite the variation from one device to another, agiven microprocessor may be quite consistent at a given temperature,with the watchdog timer varying by only a few microseconds from one tagtransmission to the next. Thus, the watchdog timer does provideconsistent signal timing that may be forecasted by an interrogator basedon knowledge of the randomization algorithm.

[0066] For more predictable performance, in another illustrativeembodiment, the watchdog timer may be calibrated. Calibration may beaccomplished in firmware in the tag by running the watchdog timer at thesame time as the tag microprocessor and using the microprocessor tocalibrate the actual characteristics of the watchdog timer. Thiscalibration process may be repeated periodically to account for factorsthat may cause the watchdog timer characteristics to drift, such aschanges in ambient temperature.

[0067] The method used to select a pseudorandom number may involvestoring a lookup table in the microprocessor's memory when the tag isprogrammed. For example, a sequence of 32 4-bit values may be placed inthe tag microprocessor's memory. These values may be used by the tagmicroprocessor to select a sequence of 32 pseudorandom values. Expandingon the example above: Set watchdog timer at 72 milliseconds; index = 0;While (true) { //Select a pseudorandom number from 0 to 13. Repeat index= ((index + 1) mod 32) until LookupTable[index] <= 13; //Wait for apseudorandom period of 63 to 76 watchdog timer intervals For i = 1 to(63 + LookupTable[index]) {sleep for 72 milliseconds}; Transmit tagdatagram;}

[0068] In the example above, the value of index may be transmitted withthe tag datagram. Similarly, if the watchdog timer is calibrated thisinformation may also be included in the tag datagram. Such informationmay improve the speed with which an interrogator or associated softwaremay synchronize with the tag's pseudorandom transmission pattern. Evenwithout such data, the interrogator or host software may synchronizewith the tag's transmission pattern (or otherwise forecast the tag'stransmission times) with only a copy of the tag's lookup table and thetag parameters that drive the tag's use of the lookup table.

[0069] As one example, the lookup table may be generated by firstchoosing 512 random sequences of, for example, 32 4-bit values. Next,the sequences may be split into 256 “low” sequences and 256 “high”sequences. The sequence used by each tag may be a combination of the lowand high sequences based on the tag's unique ID. The low sequence may beselected by using the tag ID's low order byte. The high sequence may beselected by the high-order byte of the low-order word. For example, ifL[j] is the j^(th)element of the low-order sequence, and H[k] is thek^(th) element of the high-order sequence, they may be combined into aresulting sequence R as follows: R[j]=(L[j]+H[k]) modulo 16. Softwareassociated with the interrogator or other portion of the wireless tagidentification system may thus derive the tag's lookup table based onthe 512 base sequences combined with the low-order word of the tag's ID.

[0070] To generate the set of 512 sequences, an algorithm may be usedgenerate 512 random sequences. For improved performance, a softwareprogram may test all possible pairs to reject pairs of sequences thatare similar. For example, to test a pair of sequences, the two sequencesmay be aligned and tested to ensure that they are not largely identical.A pair of sequences may be rejected if, for example, the test softwarefinds a run of 8 consecutive identical values. The test of a pair ofsequences may be complete when they have been rotated through allpossible relative positions. When the algorithm finds an unacceptablepair of sequences, it may discard one or both of the sequences in theoffending pair, replace it with a newly generated random sequence, andtry again. The result of such a process is a set of sequences withbetter than random behavior relative to one another.

[0071] Optimizing the Tag Search Procedure for the Wireless TagIdentification System

[0072] Another aspect of the invention involves optimizing a searchprocedure, if any, used by the wireless tag identification system toidentify the presence of tags. For example, PinPoint LPS uses a tagsearch procedure to identify the presence of a tag, followed by furthercommunication with the tag to receive data used to determine theidentity and location of the tag. In one illustrative embodiment, awireless tag identification system may use a first technique, e.g., asearch procedure to ascertain whether it is likely that a tag is inrange. Once a tag is detected, the system may switch to a secondtechnique, e.g., a data collection mode where the interrogatordetermines the tag's location, ID, and other data as required. Thesearch process may be essentially continuous whenever the interrogator'sDuty Cycle is On, while data collection occurs only when a tag (or atag-like signal) is found. The section below describes illustrativesearching techniques that may avoid unnecessary use of the radio channelor other wireless communications space during the search process, and/orgive users an opportunity to balance performance vs. channel usage.

[0073] In the specific case of PinPoint's LPS, each interrogator isdesigned to support up to 16 antennas. When the system is searching fortags, it cycles quickly among those antennas, with each antenna using aslot of about 19 microseconds. If all antennas are in use, then one ofthe 16 antennas is always operating. When fewer antennas are actuallyconnected to the interrogator, a slot emerges that is approximately(19*(16-N)) microseconds long (where N=number of antennas). (In analternative operating mode, the system does not leave a slot and simplyconducts the search more frequently. This somewhat improves theprobability of seeing a tag, at the cost of increased use of the radiochannel and possible interference with WLAN communications.) TilePinPoint LPS tag header requires 31 19-microsecond slots to account forthe possibility of 16 antennas for each interrogator. Sixteen slots areneeded to search 16 antennas, and 15 more slots are needed to read thedistance from the remaining 15 antennas in case the tag is first foundby the 16^(th) antenna. For instance, the tag might turn on just beforethe 16^(th) slot, and thus the interrogator may need to check theremaining 15 slots as illustrated in FIG. 3.

[0074] In the standard configuration, PinPoint's LPS uses ahigh-reliability method to ascertain whether a tag is in range of anantenna. It checks each antenna for 19 microseconds, which is enoughtime to transmit six full 127-chip sequences. The first sequence isdiscarded, as it is needed for the radio system to lock onto the AGC.The next four sequences are correlated and averaged. The final sequenceis ignored, as there is no pseudonoise data following to give propercorrelation. This approach provides a high reliability method fordetermining whether any tags are in range of an antenna.

[0075] Alternative methods may achieve somewhat less reliable LPSperformance, but with substantially less use of the radio channel. Forexample, a full search procedure that includes a plurality of sequencesmay be aborted if a correlated magnitude or other parameter of a signalreceived in a first sequence is below a threshold. In one LPSimplementation, each antenna requires about one sequence—or about threemicroseconds—for the radio automatic gain control (in the tag andinterrogator) and the carrier recovery circuit (in the interrogator) toself-calibrate. Once this is accomplished, a single sequence may be usedto determine whether a tag may be in range. If the correlated magnitudeof this single sequence is above a fixed threshold, which may be setrelatively low, the interrogator continues to read the additional threesequences and average them. However, if the first sequence is not abovethe fixed threshold, the process may abort after two sequences (of aboutthree microseconds each) and the search may move to the next antenna. Ifno tags are in range, the result may be a search process that takes lessthan seven microseconds per antenna, instead of 19 microseconds perantenna. In a system with N antennas, a slot may emerge that isapproximately ((19*16)−(7*N)) microseconds long, thereby providing awindow within which the WLA wireless communications may take place.

[0076] In another illustrative embodiment, the first test sequence mightbe four 31-chip sequences, instead of one 127-chip sequence. Thesubsequent three test sequences may remain as 127-chip sequences,providing most of the benefit of the four test sequences discussedabove. The threshold may be tested on the first 31-chip sequence. Thisshortens the search process to approximately four microseconds perantenna, and a slot emerges that is approximately (19* 16)−(4*N))microseconds in length. As an additional benefit, the use of a 31-chiptest sequence provides a means of low computational complexity todetermine the approximate tag location (modulo 31 chips). Thisinformation may be used to reduce the computational complexity of the127-chip correlation process in long-range applications by eliminatinglarge ranges of tag locations from consideration.

[0077] In this illustrative example, the analysis assumes that 16antennas are used and thus that a new search is initiated by theinterrogator every 16 slots, or about every 16*19=304 microseconds. Notethat the methods described above may take variable amounts of time tocomplete, depending on the data encountered. Still, as long as a searchis initiated and completed every 16 slots, no tags should be missed. Ifthe distance is being read from fewer than 16 antennas, the search mayin fact be initiated less frequently than every 16 slots.

[0078] While receipt of a tag signal at a single antenna may besufficient to identify the presence of a tag, reads from multipleantennas may be needed gather sufficient data to determine tag location.Thus, in contrast to some of the embodiments described above, a subsetof antennas, rather than all antennas in the wireless tag identificationsystem, may be used to search for tags; once the tag is detected by oneof those antennas in the subset, additional antennas may be used to fixthe tag location. Depending on which antenna detects the tag in thesearch process, it may be best to use only nearby antennas for fixingtag location, thus using less of the RF channel and effectivelyfiltering out unwanted signals from far-away antennas that may interferewith WLAN communications.

[0079] For example, a user or the wireless tag identification systemunder software control may decide to include a subset of antennas in tagsearches, with additional antennas used to help determine location oncethe tag is found. The antennas involved in determining location may varyaccording to where the tag was found, excluding far-away antennas fromconsideration, resulting in reduced use of the spectrum, limiting use ofthe spectrum to areas where a tag is detected, increasing the timeinterval between searches, and reducing the probability ofmisinterpreting one tag's header as part of another's.

[0080] Another approach for creating slots between searches, e.g.,within which the WLA wireless communications may take place withoutinterference, is to extend the length of the tag datagram header. Thismay increase the number of collisions between tag transmisstions anddecrease battery life. With regard to collisions, the probability of aparticular tag being seen on a given transmission is:

Y=[(T _(period)−(2×T _(chirp)))/T _(period)]^(N−1)

[0081] where Y=Yield, T_(period)=Time between transmissions,T_(chirp)=Length of transmission, and N=Number of tags. Increasing thelength of the header increases T_(chirp), and reduces Yield accordingly.This is illustrated in FIG. 4, in which only two tags are considered. Ifmore than two tags are transmitting, then the window for transmittingwithout a collision may be correspondingly smaller.

[0082] In another illustrative embodiment, tags may have differentheader lengths. For example, low priority tags may have a short header,while high priority tags may have a longer header. As one example, ifhigh priority tags have 3-millisecond headers, these tags can be foundreliably with a search process that is initiated every 2.7 milliseconds(assuming a 0.3 millisecond search process). In this example, lowerpriority tags, with headers of the minimum length, will be seen onlyabout 10% of the time. A randomized timing component described above maybe employed to avoid synchronization with other periodic processes. Iflow priority tags need to bc read reliably at portals and the like,antennas in those areas may operate at a higher duty cycle toaccommodate the shorter header length of some tags.

[0083] In another illustrative embodiment, a header for tags may beadjusted depending on whether the tag is in motion or not. For example,a tag may include a motion detector. When the tag is stationary, a shortheader may be used. When the motion detector indicates that a tag is nowin motion, the tag may use a longer header. Thus, tags in motion aprovided with a greater chance of being detected than stationary tags.For tags that are likely to be immobile for long periods of time, thistechnique may increase the window for WLAN transmission withoutinterference, or minimal interference.

[0084] In another illustrative embodiment, tags with long headers mayoperate in a detection mode followed by a transmit mode. For example,tags with long headers may turn on in two stages. First, the tagreceiver circuit may be enabled for the purpose of measuring the levelof incoming 2.4 GHz energy and other maintenance functions. Onceincoming energy is detected above a certain threshold, the morepower-hungry output amplifiers may be enabled and the tag may begin itsnormal transmission process. Toward the end of the header, thetransmitter may be enabled on some percentage of transmissionsregardless of incoming power level, in case the threshold is set toolow. The settling time assumed by the interrogator (which, as notedabove, may be one full sequence or three microseconds) may need to beadjusted upward to account for time for the tag to change modes.

[0085] In another embodiment, a first set of wireless signal frequenciesoutside of frequencies used by at least one access point in the WLAN maybe used to identify the presence of a tag. Once the presence of a tag isidentified, a second set of frequencies that includes a frequency usedby the WLAN may be used to communicate with the tag, e.g., to determinethe identity of the tag and its location. For example, lower chippingrates with narrower output filters may be employed in the search processas compared to the tag data collection process. Direct sequence 802.11WLAN communications are designed to use about ⅓ of the 2400-2483 GHzband, using an 11-megachip rate. This, for example, enables three 802.11access points to co-exist in a same coverage area or to have overlappingcoverage areas. An LPS search process that similarly uses about a thirdof the spectrum may be configured to use different parts of the spectrumthan do one or two nearby 802.11 access points. Once a tag is found, thefull spectrum may be used to verify the tag's operation, location, andID. For applications that are very sensitive to cross-interferenceissues, the complete LPS operation may be limited to the sub-band byusing a lower chipping rate and narrower filters, but sacrificinglocation accuracy and Signal to Noise Ratio (SNR).

[0086] For even narrower band operation, such as to minimizeinterference with frequency hoppers, it is possible to use a frequencyhopping radio to search for tags. If energy is detected 3.36 GHz higher,a tag may be in range. In this event, the system may switch to morereliable methods for verifying the tag's operation, location, and ID. Aspecial signal need not be transmitted by the LPS to search for the tag;instead, the tag may transpond normal WLAN traffic. If this is the case,then a tag may be detected even when the interrogator/antenna Duty Cycleis Off. Thus, Case 4 in the previous section becomes Case 3; althoughthe interrogator Duty Cycle is Off, if a tag transponds normal WLANtraffic, energy may be detected in a band 3.36 GHz higher, suggestingthat the tag is present. The interrogator may also extract the tag IDfrom transponded WLAN traffic, particularly if the tag is amplitudemodulated. Alternatively, if there is no 2.4 GHz energy on the channel,a very short burst of energy may be emitted periodically from theinterrogator to test for the probable presence of transponding tags.Once a tag is detected in range, a longer 802.11 packet or LPS packetmay be sent to estimate tag location and collect the tag data.

[0087] Integration of a WLAN with Wireless Tag Identification SystemOperation

[0088] In previous sections, various techniques are described tominimize a wireless tag identification system's use of a wirelesscommunication channel(s) in a manner that is independent of WLANoperation. To achieve a desired level of guaranteed WLAN performance,the LPS may be configured with substantially degraded operationalparameters. For more optimal, shared utilization of the wirelesschannel(s), some kind of integration between the wireless tagidentification system and the WLAN may be advantageous.

[0089] In general, three approaches are discussed below for integratingLPS and WLAN operation:

[0090] 1. Clear channel assessment: The LPS may monitor the wirelessenvironment and may infer information about WLAN operation. The LPS maythen time its operation to periods when a WLAN is not using the radiochannel(s).

[0091] 2. Communication with WLAN: The LPS may be in communication withthe WLAN and/or its access points, either directly or through hostsoftware, and commands may be sent between the devices to coordinate useof the wireless channel(s).

[0092] 3. Combined LPS and WLAN: An LPS and a WLAN device may beintegrated as a single device or two tightly integrated devices,potentially using a shared microprocessor and/or a shared radio.Operation of the two devices may be tightly integrated.

[0093] Clear Channel Assessment

[0094] In one illustrative embodiment in accordance with one aspect ofthe invention, the LPS may monitor the wireless environment at least inthe vicinity of one interrogator or antenna, and infer information aboutWLAN operation. The LPS may then adjust the timing of its wirelesssignal generation to use the open slots in the WLAN protocol. Oneadvantage of this technique is that it may be accomplished entirely onthe LPS side, without modification of the WLAN, and may enable a widevariety of LPSs to operate harmoniously with a wide variety of off-theshelf WLAN products.

[0095] In a typical WLAN operating under the 802.11 standard, after eachtransmission, the 802.11 protocol defines a series of slots, each 20microseconds in length. When two or more wireless communication devicescheck a slot and find it clear, they may begin transmission. If bothradios are in communication with the same access point, perhaps one ofthese transmissions will be correctly received; it is also possible thatboth will be corrupted. In any case, correctly received transmissionswill normally be confirmed with an acknowledgement from the accesspoint. A unit that fails to receive an acknowledgement may retry, butmust back off to later slots on a randomized basis according to rulesdefined in the 802.11 protocol. CCA is the nomenclature used in 802.11for “Clear Channel Assessment.” Before transmitting, 802.11 radios checkthe radio channel for a few microseconds to see if the channel is clear.A busy channel is taken to indicate that a higher-priority 802.11 deviceis using the channel, and a back-off procedure is employed. Three typesof CCA checks are allowed in the Direct Sequence version of 802.11.First, the CCA may simply check for received power at 2.4 GHz. Because ahigher-priority radio may be quite far away, a low threshold is set.Second, the CCA may look for a correlated signal. Third, a combinationof these methods may be used.

[0096] An LPS interrogator may search for tags by emitting a wirelesssignal into a detection area, and searching for tag responses to thisenergy. For interrogators operating in the 2.4 GHz band, this search isimplemented by emitting 2.4 GHz energy into the detection area. Forpassive (non-battery) LPSs, the 2.4 GHz field must be strong enough topower the tag over the air. For battery-operated systems, this proximityrestriction is eliminated and a relatively extended area is interrogatedusing some form of 2.4 GHz energy. Tags may be detected when theysomehow reflect or otherwise respond to this energy.

[0097] For modulated backscatter RFID systems, the interrogation signalis typically narrow band; in the 2.4 GHz band, frequency hoppers aretypically employed. (Although frequency hoppers are spread spectrumdevices, at any given moment they operate as narrow band emitters.) Whenthe tag is enabled, it reflects this energy into the sidebands. The tagmay be read by detecting energy in these sidebands.

[0098] In the PinPoint LPS, a wide band (40-megachip direct sequence)interrogation signal centered at 2.44 GHz is up-converted to 5.80 GHz bythe tag and transmitted back to the interrogator. The tag is read bydetecting and processing this 5.80 GHz response signal.

[0099] Radios operating in the 2.4 GHz band are spread spectrum devices,and as such, several such radios can operate simultaneously. Thus, undermany operational conditions, an LPS may transmit data harmoniously atthe same time as a WLAN. That is, in many cases those transmissions willsucceed even while the other system continues to operate. Severalapproaches on the LPS side, outlined in previous sections of this paper,may be used to keep the channel clear a large percentage of the time.However, in cases where the two systems operate simultaneously andinterfere with each other to some degree, integrated operation may beadvantageous for best performance.

[0100] One way to accomplish coordination between an LPS and a WLAN isto add CCA functionality to the LPS. An LPS CCA 8, such as that shown inFIG. 2, may include special hardware that monitors the 2.4 GHz radiochannel, looking for abrupt increases or decreases or other changes inwireless signal energy. Any suitable hardware, software or othercomponents may be used in the LPS CCA 8, and may be identical, orsimilar, to that used in WLAN CCAs. The information from the LPS CCA 8is sent to the LPS controller 7, one or more interrogators 6 or otherportions of the wireless tag identification system as appropriate. Thus,the LPS CCA may communicate with the entire LPS through the LPScontroller 7, or only to one or more interrogators 6 or antenna modules5 as appropriate. An abrupt increase in energy may indicate that a WLANtransmission has begun, while an abrupt decrease in energy may indicatethat the transmission has completed. For example, an LPS CCA 8 maydetect 802.11 packets as increased energy for about a half-millisecond.After a decrease in energy, indicating WLAN packet completion, the LPSmay wait for a fixed period of time, for example, one millisecond. If nopower is detected during this period (other than very short spikes,which are not typical of WLAN operation), the WLAN may be taken to be“quiet,” and LPS operation may begin for a period of time. Theinformation supplied by the LPS CCA 8 may be used to control theoperation of all antennas 5 and/or interrogators 6 in the LPS, or only aselected set of antennas 5 and/or interrogators 6 since the WLAN trafficdetected by the LPS CCA 8 may only need affect a portion of the LPS, notthe entire system.

[0101] As an example, an interrogator 6 may be on the following dutycycle: 50 milliseconds On, 25 milliseconds Off (fixed), plus 0-50milliseconds Off (randomized). This results, on average, in a 50% dutycycle, with 50 milliseconds On and 50 milliseconds Off. At the end ofthe period, a CCA-enabled interrogator (an interrogator incorporating orin communication with an LPS CCA) may or may not simply turn On blindly.Instead, it may check the channel and wait until it is clear for aperiod of time, such as one millisecond. This period is called the LPSCCA duration. Once a clear channel is detected, the interrogator mayturn On. This CCA procedure is illustrated, for example, in FIG. 5. Asdescribed above, each “slot” after an 802.11 packet is 20 microsecondsin length; thus, a one-millisecond CCA duration gives 802.11 devices 50slots in which to initiate transmission. This effectively gives the WLANpriority over the LPS. A lower CCA duration, such as one randomlyvarying between 50 and 100 microseconds, may tend to increasingly favorLPS operation. The LPS CCA duration may be set and/or adjusted by a useror automatically by the LPS based on WLAN operational details,cross-interference performance requirements and/or actual WLAN/LPSperformance. For example, if the WLAN experiences unacceptable delays intransmission, a signal indicating the disruption may be sent to the LPS,which may respond by adjusting the CCA duration.

[0102] The time spent finding a clear channel may be incorporated intothe randomized Off period for interrogators/antennas. In an example ofthis technique, the time spent finding a clear channel in one cycle maybe subtracted from the randomized Off duration in the next cycle,resulting in the desired average duty cycle.

[0103] In one embodiment, if the LPS CCA entirely blocks LPS operation,the LPS CCA may time out and allow transmission to proceed regardless ofdetected WLAN traffic or signals that indicate WLA wirelesscommunication is ongoing. This approach may be appropriate forinstallations where WLAN operation is characterized by relatively shortbursts of activity, in which case the timeout probably indicates thatsome other interference mechanism is triggering the LPS CCA.

[0104] Alternatively, the LPS CCA duration may be adjusted dynamicallyto achieve a pre-defined level of “acceptable” interference with eitherthe WLAN or LPS wireless operations. For example, the LPS CCA durationmay be configured to start at 1000 microseconds, but with a required 50%success rate, i.e., the percentage time that the LPS is permitted totransmit wireless signals is about 50%. If this LPS CCA duration blocksLPS wireless operation more than 50% of the time (before it times out),the LPS CCA duration may be reduced until the LPS times out only 50% ofthe time. Note that in 802.11, an LPS CCA duration of less than 25microseconds may interfere with high priority WLAN transmissions such asAcknowledgements, and a minimum LPS CCA duration may be set to avoidsuch interference.

[0105] Numerous physical implementations of an LPS CCA are possible,with various possible advantages and disadvantages. One consideration isthe location of the LPS CCA. In some cases, it may be convenient for theLPS CCA to be co-located with an interrogator and/or an antenna module.This approach is the logical choice for mobile and handheldinterrogators or other tag sensors. For interrogators or antennas infixed positions, especially interrogators like PinPoint's with multipleantennas on long cables, better performance may be achieved by placingthe LPS CCA in a different location, closer to a center of WLAN activity(an area that experiences more overall wireless transmission traffic).In particular, it may be best to place the LPS CCA near a WLAN accesspoint or group of access points, on the assumption that WLAN traffic ismost relevant insofar as it is seen by the access point. Forinterrogators that support multiple remote antennas, another option maybe to incorporate an LPS CCA in each remote LPS antenna module and tochange the duty cycle of each individual antenna in reaction to localvariations in LPS CCA data.

[0106] Another consideration is the nature of the communication betweenthe LPS CCA and the rest of the LPS. For example, it may or may not bepractical to report CCA status every microsecond, because themicroprocessor or other device in the LPS CCA and/or a physicalconnection (such as twisted pair at 112 kbps) may be unable to keep LIPwith the load. Thus, in one embodiment, it may be desirable for the LPSCCA to include at least a rudimentary filtering and buffering. Oneapproach is to use an 8-bit microprocessor to check the CCA statuscontinuously and to report average levels periodically (for example,every 10 microseconds). Reports may be limited to times when thereceived energy moves above or below a preset threshold for a presetperiod of time (for example, 100 consecutive microseconds).

[0107] A third consideration is the nature of the LPS CCA detector,i.e., what is being measured and reported. Some options include:

[0108] A fixed threshold detector, where the CCA reports a one or a zerodepending on whether the received energy (i.e., wireless signal energyindicative of WLA wireless communication) is above or below a fixedthreshold. While this is a simple design, different LANs and differentinstallations may require a different threshold.

[0109] A variable threshold detector that operates in essentially thesame way as a fixed threshold detector, except that the threshold is seteither at installation or on command from the LPS.

[0110] A CCA circuit using complete or partial chip sets used in WLANproducts. For example, Intersil sells a complete 802.11 chip set on aPCMCIA card. Rudimentary software may be written to check a channel forcorrelated signals; more sophisticated software may track the packetsbeing sent across the network. This approach has two importantlimitations:

[0111] 1. For different (not 802.11) WLAN protocols, the CCA degeneratesto the performance of a threshold detector.

[0112] 2. When multiple WLANs are running, each in a sub-band of2400-2483 MHz, the CCA may only track one of the sub-bands. However,custom chips with three detection circuits may be used to track threesub-bands, for example.

[0113] Despite these limitations, it may be desirable to use 802.11chips for CCA detection, especially as the cost of these chips decrease.This may provide an opportunity for closer integration with 802.11through software upgrades to the interrogator, LPS CCA or other portionsof the wireless tag identification system. In one implementation, wherethe LPS CCA is in fact a full 802.11 radio, the same circuit may be usedto transmit information back to the LPS or other system over the air.

[0114] The use of a CCA to coordinate the performance of a WLAN and LPSmay be complicated somewhat by the possibility that multiple WLANs (upto three or four) may co-exist nearby, each using different channels.Thus, LPS performance may need to be coordinated not only with a singleWLAN, but with more than one WLAN. For example, an LPS may be connectedwith one WLAN, and a wired connection (shown the by dashed line betweenthe WLAN controller 40 b and the LPS controller 7 in FIG. 2) may be usedto coordinate the performance of the LPS and this first WLAN. However,there may be one or more additional WLANs operating nearby that areunconnected to the LPS, as also shown in FIG. 2. One way to coordinatethe performance of multiple systems, such as in the case described, maybe to use multiple CCAs to monitor different channels. In the embodimentillustrated in FIG. 2, the two WLANs use different channels. A CCA (notshown) may be included in the WLAN, and CCA information and othercoordination information may be sent between the LPS and the WLAN over ahard wire connection to coordinate the performance of the LPS and theWLAN. The LPS CCA 8 may be used to coordinate the performance of the LPSand the second WLAN that does not have a direct communications link tothe LPS. The LPS may use the CCA information both from the WLANcontroller 40 b and the LPS CCA 8 to control wireless operations of theLPS.

[0115] Communication Between a WLAN and a Wireless Tag IdentificationSystem

[0116] For tighter integration between a WLAN and a wireless tagidentification system, software and/or firmware may be used to share oneor more channels cooperatively. For example, in one illustrativeembodiment, if the WLAN is being used for real-time voice and/or videocommunication, fast response may be needed for interactivity. Still,latencies of a fraction of a second may be tolerable in manycircumstances. In such situations, the WLAN may buffer transmissions inorder to leave slots for LPS operation, such as 10 milliseconds inlength. Particularly in cases where content is streamed from a host tohandheld devices through a wireless access point, the sharing of thechannel may be orchestrated from the data source.

[0117] Channel sharing may be handled by one-way commands from the WLANto the LPS. Thus, the WLAN may indicate to the LPS when the LPS ispermitted to transmit wireless signals based on actual or expectedwireless communications within the WLAN in a particular area or overallwithin the WLAN. The WLAN may take LPS operations into account, e.g., bypermitting the LPS to transmit wireless signals at least after a certainmaximum time interval. Such one-side control may, however, createproblems in asset location in some cases. For example, an LPSinterrogator and/or host software may forecast tag transmission timesand use the forecasted times to identify tags and their location. If aparticular tag has not been seen for an excessive period of time, theLPS may forecast the next transmission time of the tag and adjust theduty cycle for one or more interrogators or antennas to receive the nexttransmission from the missing tag. However, the LPS may be restrictedfrom making such adjustments if another system (e.g., the WLAN) isblindly controlling its duty cycle via one-way communications.

[0118] Alternatively, two-way communication between the LPS and WLAN maybe advantageous in some circumstances. In order to control cooperativesharing of a channel, the WLAN software and the LPS may exchangecommands. One approach is to use a token to pass control of a channel.Another is a Master-Slave design, where one system (Master) sendscommands to another (Slave). The Master may be the LPS, the WLAN system,or a third device (e.g., a host system 10 shown in FIG. 2) that sendscommands to both. In a Master-Slave design, the Slave may employ a meansto request use of the channel for high-priority communications. Forexample, if the WLAN is the Master, the LPS may request permission totransmit wireless signals at a particular time so that it may identify amissing tag. In some cases, it may be preferable to use an open softwareAPI for system coordination, so that a WLAN application may be able toadjust the LPS duty cycle to accommodate bursts of WLAN activity.

[0119] Such coordination may be accomplished through wirelesscommunication between the LPS and the WLAN or through a wired network.One advantage of the wireless approach is the precise sharing of a timebase. For example, a command might take the form, “You have control ofthe channel for exactly the 10 milliseconds following this packet.” In awireless system, the exact receipt time (except for radio propagationtime) is known. In a wired system, various latencies may make itdifficult to predict the exact receipt time, especially if the messageis transmitted using Windows NT or similar operating system services.Nonetheless, it may be more practical to control the duty cycle byexchange of normal network packets through whatever means are easilyavailable, at the cost of some latency during the data transmissionperiod. A hardware-independent approach may enable an applicationprogram (such as the one that originates video) to orchestrate sharingof wireless communications channels in a way that does not requirespecial hardware or firmware on the WLAN.

[0120] In another embodiment, system integration using SNMP may providea means to adjust the LPS duty cycle. Periodic SNMP queries to wirelessaccess points in range of an interrogator may be used to ascertain boththe level of WLAN traffic and the level of networking errors. Thisinformation may be used by the LPS to adjust the duty cycle to accountfor minute-to-minute changes in network utilization. When WLAN use isrelatively stable over time, SNMP information may be used to search forthe highest level of LPS performance that does not appreciably degradeWLAN performance. SNMP may also be used to detect sudden increases innetwork use, providing information for the LPS to back off from channeluse. For periods where the LPS is unable to sustain target levels ofperformance without unacceptable degradation of WLAN performance, thisinformation may be logged with a timestamp.

[0121] Combined Wireless Tag Identification System and WLAN

[0122] In some cases, an interrogator or antenna module may beintegrated with a WLAN as a single device or two tightly coupleddevices, potentially using a shared microprocessor and/or a sharedradio. This approach may have several advantages:

[0123] Installation Logistics:

[0124] Installing cable and hardware may result in costs andinstallation delay. The number of hours required for installation of aWLAN or LPS is usually not large. Nonetheless, site planning, managementof the installation process, and the logistics of bringing qualifiedinstallers on-site can result in significant cost and delay,particularly if specially licensed labor is required. If the customerchooses to include multiple functions in such installations, only asingle installation is required (at least in some areas) rather thanmultiple separate installations, thus decreasing the time, cost, andplanning required.

[0125] Shared Network Connection:

[0126] Both a WLAN and a high-capacity LPS can generate substantial LANtraffic. For this reason, they are sometimes placed on their ownsubnets. In any case, the systems need physical LAN connections. In someinstallations, the LAN upgrade is a substantial part of the installationundertaking and cost. Again, this factor can be minimized by a sharedinstallation.

[0127] A special case of the shared network concept involves theinterrogator or antenna module acting as a device on the WLAN,communicating wirelessly to the LPS controller 7 or other host systemthrough an access point. Thus, cabling may be run to fixed WLAN accesspoints, and the interrogators or antenna modules may communicate data tothe host through those access points. This may be particularly helpfulin outdoor installations or for mobile interrogators. The access pointmay be installed in a convenient fixed position, providing networkconnectivity to any WLAN radio in range. This allows interrogators inrange to bc used wherever they are needed, communicating with the LPScontroller through a WLAN connection. As noted previously, the WLANhardware in the interrogator may double as an LPS CCA.

[0128] Certain types of WLANs, such as medical telemetry systems, feeddata to a host through a proprietary wired network. A PinPoint LPSinterrogator includes a processor running the Linux operating system andmay double as a WLAN access point for the telemetry system.

[0129] Shared Hardware and Software:

[0130] From a cost standpoint, the more hardware that can be shared bythe WLAN and the LPS, the better.

[0131] Many LPS installations feed data from interrogators into a hostcomputer, such as the LPS controller 7. Some WLAN applications alsotransmit all or most data through a host computer, such as the WLANcontroller 40. Medical telemetry systems are a good example of this. Ifsuch a host computer can be shared, savings may be realized on the costof host hardware, software, and support.

[0132] To ensure that network traffic is isolated, in some installationsit may be preferable to put an LPS and a WLAN on a subnet. This subnetmay be shared between these two functions.

[0133] Even if different radios are employed for WLAN and LPS wirelesscommunications, there may be advantages to including LPS and WLANcapabilities in the same device, as in the aforementioned example of ahandheld interrogator. For a long-range interrogator such as an LPSinterrogator, there may be significant advantages to including bothfunctions in one package. For example, in the home or office, there ismuch appeal for a single device that can both provide wireless accessand keep track of assets in range. Plastics, power supply, networkconnection, back plane, circuit boards, microprocessor, packaging, andmarketing may be shared even if the radios are different. The radiosthemselves may be configured as plug-in components, so the user canselect which technologies to use in a given situation, providingflexibility and an upgrade path.

[0134] Once the two radios are tightly integrated, potentially under thecontrol of a single microprocessor, there is an opportunity to share theradio channel more efficiently. The same techniques described previouslymay be employed, but with negligible latency time to switch between tagsensor and WLAN modes. If the WLAN and the LPS operate at differentfrequencies, they may be integrated and may operate simultaneously. Forexample, Medical Telemetry systems (VHF and UHF television bands) andcordless phones such as DECT (1.9 GHz) usually operate at frequenciesthat are different from LPSs. If the unit is designed with reasonablecare, the two systems should not affect each other's operation.

[0135] As shown in FIG. 6, a tag sensor 3 may be designed using 802.11components. (The tag sensor 3 is given the same reference numeral as anaccess point since the tag sensor in this embodiment may also operate asan access point. In fact, the tag sensor may be installed as part of theWLAN and function as an access point.) A standard 802.11 radio module 31in the interrogator may be used to transmit a Direct Sequence SpreadSpectrum signal in the 2400-2483 GHz band. The tag 2 may up-convert thissignal to the 5725-5875 MHz band by mixing the incoming signal from areceive antenna 21 at a mixer 25 with a 3358 MHz tone from an oscillator26. The choice of 3358 MHz is optimized for up-conversion from thccenter of one band at 2442 MHz to the center of the other band at 5800MHz. 802.11 signals are not necessarily centered at 2442 MHz;nonetheless, the tag filtering is such that only a few dB are losttoward the edges of the 2400-2483 MHz band. A signal in the 5725-5875MHz band may be returned to the tag sensor 3 and received by a 5800 MHzantenna 34. In the tag sensor 3, the 5800 MHz signal from the tag 2 maybe mixed with another 3358 MHz tone in a mixer 33, resulting in therecovery of the center frequency originally transmitted by theinterrogator. Note that circuitry from the tag may be reused toimplement the 3358 MHz oscillator 35 and the mixer 33. The result ofthis mixing is a signal that is identical to that originallytransmitted, which may be fed into a second Receive 802.11-based radio32. It is best, but not necessary, for the 802.11 transmit and receiveradios to share a frequency source, thus eliminating a potential sourceof carrier frequency variation. Likewise, it is desirable for the 3358MHz oscillator 35 in the receiver to use a higher quality frequencysource than would be found in an inexpensive tag. Thus, the main sourceof carrier frequency variation would then be in the tag itself.

[0136] The tag sensor's 3 transmit/receive radio may use exactly thehardware of a full 802.11 transmit/receive radio. This radio may operatein four modes. First, it may operate as the transmit portion of the tagsensor 3. Second, it may operate as an 802.11 1) transmit/receive radio,i.e., as a radio for an access point 3. Third, it may transmit data toanother 802.11 radio, while at the same time the signal is transpondedby a tag. Finally, it may receive data from another 802.11 radio at 2.4GHz, while at a slightly delayed time a transponded version of the samesignal same signal is received from a tag at 5.8 GHz, as shown in FIG.7.

[0137] Not shown in FIG. 6 is the tag modulator. Periodically, such asevery two microseconds, on an asynchronous basis, the tag 2 may invertthe phase to signal a zero bit (or by opposite convention, a phaseinversion may signal a one bit). If the phase happens to be inverted inthe middle of an 802.11 sequence, the result may be little or nocorrelation in the signal received by the LPS reader. If the phase isinverted near the transition from one sequence to the next, bothsequences may be correlated, but a processor decoding this data will befaced with a phase inversion imposed on top of the 802.11 protocol. The802.11 protocol supports three types of signaling, BPSK, QPSK, and CCK,all using an 11-MB chip rate. In each case, an asynchronous phaseinversion will cause one of two conditions, either a 180° change ofphase between two sequences, or a lack of correlation followed by a 180°change of phase. In both cases, the phase change may be detected bynoting a discrepancy between the transmitted data vs. the received data.The same principles apply for the data signaling to be used by futureversions of 802.11 in the 5 GHz band, although with a differentfrequency plan for the tag and receiver used for operation.

[0138] A system that closely integrates an 802.11b wireless LAN with thefunctionality of a wireless tag identification system should be ableboth to locate high volumes of tags and provide high-speedcommunication. The tightest integration between both systems may beprovided when the data structures of the WLAN communication system mayalso be used for location purposes. Such a system is described below.

[0139] The essence of an integrated system may lie in providing hardwareand protocols that allow the asset location element of the system to usethe WLAN traffic to obtain data on the location of tags. To do this, thebasic system elements in both the WLAN data and the LPS location framesshould be commensurate; that is, they may both occupy the same amount oftime. Furthermore, the data structure for the LPS element may be shortenough that the shortest possible WLAN packets are able to provideuseful information when they illuminate the tag. Additionally, thehardware may be structured so that normal WLAN transmissions may be usedin the LPS elements for location. The following description, inconjunction with the figures provided, illustrates a system that mayprovide not only coarse location data from standard 802.11b data frames,but may also provide finer resolution by using enhanced data frames.

[0140] A combined WLAN/LPS may include a standard 802.11b access point.Commercially available access points have two major hardware elements: asection dealing with controlling the media, forming on-air data framesfrom wired LAN traffic and forming wired LAN frames from on-air wirelesstraffic, referred to here as the media access controller (MAC), and asection dealing with the physical modulation of data onto an RF carrier,amplification, switching to the antenna, receive amplification anddemodulation. FIG. 8 shows a schematic block diagram of an 802.11baccess point 3. Also shown is a standard 802.11b mobile device 41(station) sending and receiving WLAN traffic in the normal way. Theemissions from the 802.11b access point 3 may also illuminate any LPStags 2 in the vicinity. The tags 2 may convert the incoming signals at2.4 GHz to signals at 5.8 GHz (with added, slower modulation containingthe tag's data).

[0141] The proposed system in this illustrative embodiment mayincorporate an additional element in a separate receive chain: adownconverter 42 that takes the incoming signals from an LPS tag 2 at5.8 GHz and shifts the frequency to 2.44 GHz. The incoming signals maythen be processed with chipsets designed for 802.11b signals. FIG. 8shows schematically that these signals are processed through a 2.4 GHzLNA/downconverter 42, then through an RF receiver section 43, an IFsection 44, and then through an IF baseband section 45. Theaforementioned functions may be accomplished with commercially availableparts in some cases, provided that these parts have sufficient bandwidthto accommodate the signals being processed. Examples of commerciallyavailable parts that have the proper architecture to process DSSS802.11b signals include Intersil HFA 3424 (LNA), HFA3624 (IFdownconverter) and HFA 3726 (baseband downconverter). These three blocksmay essentially duplicate the receive functions of an 802.11b accesspoint.

[0142] The hardware may incorporate an additional transmit chainelement, the wide-bandwidth LPS Tx chain element 46 consisting of amodulator and upconverter (not shown) to replicate the functions in theTx chain in the standard access point 3, but with greater bandwidth tosupport higher resolution LPS location determination. Finally, thehardware may incorporate a digital signal processing element 71 specificto the LPS functionality that uses signals available in the standard802.11b MAC. These signals include (but are not limited to) transmitsync, transmitted data, clear channel assessment and some indicator thatthe access point has no data to send, thus allowing the LPS elements touse the channel.

[0143] The system concept is that the LPS elements in the receive chainmay use standard 802.11b signals to locate an LPS tag. These signalshave a single-sided baseband bandwidth of 11 MHz and use an11-megachip-per-second spreading signal, resulting in a base resolutionfor this mode of about 24 feet. A standard 802.11b system may have twomodes of transmitted data frame: frames with short PHY layer convergenceprocedure (PLCP) and frames with long PLCP, as shown in FIG. 9.Distances to LPS tags may be measured with coarse resolution using justthe short PLCP preamble in short PLCP frames. These preambles have fixeddata and occupy 72 microseconds when transmitted.

[0144] In the integrated system described herein, the LPS receiver mayinclude a demodulator and correlator supporting only DBPSK, which is amost restrictive case. However, a similar integrated system may bedesigned that may support higher orders of 802.11b modulation, such asDQPSK, PBCC, or CCK; if such support is included, some of therestrictions noted in this discussion may be eliminated. Note thathighly integrated 802.11b chips are available that are capable ofhandling all forms of 802.11b modulation, although the underlyingcorrelations may not be accessible depending on the detailedspecifications of such chips.

[0145] In addition, special LPS-specific data frames may be injectedinto the wireless LAN traffic when two conditions coincide:

[0146] 1. The access point has no data to send to remote stations, and

[0147] 2. The clear channel assessment signal indicates that no otherstations are transmitting.

[0148] In this case, an enhanced, but noncompliant PLCP PHY protocoldata unit (PPDU) may be sent that consists of the standard PLCP preambleand a header indicating that the originator of the current frame is theaccess point and the destination of the current frame is also the accesspoint. Remote stations will drop this frame, as the remote stationaddress will not match the destination address. However, the system maymodify the PSDU part of this frame from the standard 802.11b definition.The PSDU may consist of DBPSK data elements, each being 1 microsecond inlength. The difference is that, instead of each symbol consisting of adata bit spread with the 111-bit Barker sequence at 11 Mcps specified inthe 802.11 standard, each symbol may consist of a data bit spread with alonger sequence spread at a faster rate. One example may be where eachdata bit is spread with a 31-bit sequence (such as the one produced bythe generator polynomial [5 2] in Dixon's notation), which has aspreading rate of 31 megachips per second. This spread signal may resultin a distance resolution on the order of 8 feet, which is significantlyhigher than the 24 foot resolution provided by the short PLCP frames.Any combination of spreading sequence length and spreading rate thatresults in a symbol time duration of one microsecond will becommensurate with the long PLCP frame and may be incorporated within it.

[0149] The access point MAC may ignore this special LPS frame. One wayto do this is to use the RTS/CTS line to drop the frame arriving withthe access point's destination address when this line is valid. Thisprocedure may allow the LPS digital signal processor to determine thedistance to the tag, to decode the remainder of the datagram, to formatthe distance and tag ID properly, and to return it either to the MAC orto an Ethernet data generator. The Ethernet data generator may simplyact as a mini-hub to inject the Ethernet frame into the wired LANconnection. The LPS digital signal processor may also need to providethe MAC with the proper LENGTH and DataRate parameters in the TXVECTORassociated with the PHY-TXSTART.request primitive so that the standardMAC may respond appropriately to the incoming LPS frame.

[0150] The functionality of the LPS digital signal processor, the MAC,and the Ethernet interface may be very similar to the functionality ofcurrent commercially available standard 802.11b MAC devices. Thefunctionality of all of these elements may be combined into a singledevice that may be integrated into a single silicon chip to reduce cost.This may also simplify the interface to external circuits.

[0151] The structure of the tag signal returning to the LPS elements ofthis system may next be considered. The tag preferably supports distancemeasurement, identification, and an indication that the tag datagram hasarrived intact (i.e., was not corrupted by tag or 802.11b stationcollisions). Additionally, the tag may need to support the transmissionof low-bandwidth data from equipment attached to the tag. FIG. 10 showsa possible tag datagram structure that may accommodate these needs. Thisdatagram may consist of at least two repetitions of the basic datagram,which consists of 8 fields: the header, during which the system measuresdistance; the start-of-data marker field, to indicate where the dataportion begins; the checksum field; the ID field; the status field; andthe user data field, a stop bit field, and a framing bit.

[0152] The tag may differentially bi-phase modulate its data bits ontothe signal provided by the access point. The tag transmissions may ormay not be coherent in any way with the access point signals. The tagshould provide data bits that are longer than the transmitted symbols,because the tag data transitions may destroy a particular symbol'scorrelation in the LPS demodulator. The system as illustrated uses tagdata bit durations of two microseconds, or two transmitted symbols,resulting in a potential phase shift every second one-microsecondsymbol. This ensures at least one intact sequence received by the LPSreceiver for each bit period. Slower tag modulation would result inmultiple intact sequences per tag bit.

[0153] The tag header field may consist of two tag data bits, or fourtransmitted symbols, that become four correlation peaks in the LPSdemodulator. This may be sufficient for the LPS digital signal processorto recognize that correlations are present and to determine the distanceto the tag. The start-of-data (SOD) field may consist of three tag databits that provide a flag to indicate that the header field has ended andthe data field is beginning. One implementation of the SOD field is thebit pattern “010”. The checksum field may consist of eight tag databits. One implementation of the checksum is an 8-bit CRC using thepolynomial x⁸+x⁴+x³+x²+x⁰ as its generator. This checksum may protectthe ID, status, and user data fields.

[0154] The tag ID field may consist of 32 bits, sufficient for about 4.3billion unique tags, with the LSB transmitted first. The tag statusfield may consist of four bits for housekeeping information. Oneimplementation may assign one bit to the indication of low tag battery,one bit to the indication of tag tampering or other user-generatedaction, and one bit to indicate that the subsequent user data fieldcontains valid data, leaving one bit reserved. The tag user data fieldmay consist of eight bits of data provided by the user through a serialconnection to the tag electronics.

[0155] The transmitted data from a tag may consist of at least tworepetitions of this basic datagram. The stop bit field may be a singletag data bit whose only purpose is to allow the LPS demodulator tofinish processing before the tag turns off the RF link. The framing bitmay be implemented as a 2-microsecond period during which the tagtransmitter is disabled, thus clearly delimiting the datagramboundaries. If the transmitted data from a tag consists of just tworepetitions of the basic datagram, then the duration of the tagtransmission is 117 bits or 234 microseconds. The benefit of repeatingthe datagram during tag transmission may be seen by examining FIG. 11,which illustrates the potential timing for the system. The tags may becompletely asynchronous to the rest of the system, that is, they maytransmit at times that are independent from the transmission times ofthe rest of the system. The tags may be provided with a checksum or CRC,since two tags may transmit at the same time, which may result incorrupted data.

[0156]FIG. 11 illustrates several possible scenarios for tagtransmission and tag data reception. The transmission associated withtag #53 shows what may happen when the system begins to transmit to atag that is already on. In this case, the signal received from the tagmay occur as soon as the demodulator is connected to the LPS receive RFchain and bits start to stream out. The LPS demodulator may be unawareof the time at which the tag started transmission, so it may buffer thereceived data in a store at least two tag datagrams long and then parsethe data to see if it is consistent. If the system switches on too lateto receive any of the repeated basic datagrams, then the tag data may beignored.

[0157] Note that, in FIG. 11, the access point is illuminating tag #53with a short PLCP frame. In the most restrictive case where the LPSreceiver can only decode DBPSK signals, the LPS digital signal processormay only respond to the short PLCP preamble, which has a duration of 72microseconds. If a tag turns on exactly at the same time as the accesspoint (as shown by tag #F3), then the tag data may be decoded only forpart of the basic datagram, as 72 microseconds is only enough time toread 36 of the 58 bits in a datagram. The system may be able to measurethe distance to that tag, recognize the SOD, and decode properly theCRC, but may then be able to decode properly at most 23 of the 32 IDdata bits, since it may only be able to decode the data coinciding withthe DBPSK portion of the preamble. Since the data in the ID field may besent with the LSB first, the lowest 23 bits of the ID may enablesoftware in the LPS portion of the system to match the received CRC withthe last known good transmission from all tags to see whether that CRChas been received before. If that CRC has been received before and ifthe lowest order bits received match the ID of a tag that has been seenbefore, then the system may flag that particular read as a “probablegood” tag read and provide that information to the databases. If thereis no such match, the software may either drop the data from thedatabases or place the data in a “probable bad” table. In manyinstallations with thousands of tags, most of the tags may be stationaryfor long periods of time. Since there may be many accurate locationreads for tags sitting stationary for days or weeks at a time, the mostrelevant information may be that the tag is still there and has beenseen by the system.

[0158]FIG. 12 shows the less restrictive case where the LPS receiver maydecode tag-transponded DQPSK signals, and thus the entire short PLCPthat is 120 microseconds in length. The tag datagram that is 118microseconds in length may repeat several times; three repetitions areshown in the figure. If the PLCP is transmitted at any time during theserepetitions, a complete datagram may be received in two portions asshown. It is a simple matter to concatenate these two portions together.If the LPS receiver requires a few microseconds to lock onto the PLCPmessage, the tag datagram may be shortened accordingly, for example, byreducing the number of User Data bits from 8 bits to 6 bits.

[0159] The tag datagram may be transmitted a number of times, with twicebeing the recommended minimum. The more times the datagram is repeated,the higher the probability that the tag will be detected. Approximately8 repetitions, resulting in a half-millisecond transmission, may be areasonable trade-off between battery life, tag collisions, and thelikelihood of operating simultaneously with the asynchronoustransmission of a PLCP.

[0160] In the least restrictive case, the LPS receiver may be capable ofdecoding tag-transponded PBCC and CCK signals, in which case the entirePPDU time period can be used for detecting tags. It should be noted thatthese modulation techniques provide lower SNR than DBPSK and DQPSK,which may result in a shorter tag read range when such transmissions areinvolved.

[0161] We now examine the system operation when a tag turns on after theaccess point has been transmitting (tag #9A in FIG. 1). In this case,the LPS digital signal processor may find no correlation until well intothe tag datagram. Since the access point may be using a short PLCP PPDU,the tag data returned to the LPS digital signal processor may be only apartial datagram and may contain less than 23 bits of the tag ID. If noID can be read (i.e., the tag is still on when the short PLCP preambleends), the system may check the received CRC against its stored table ofCRCs that were received properly. If there is a match, the system mayflag the partial data as “possibly good.” If there is no match, then thetag data may be discarded. Further processing combining both locationand CRC with low-order ID bits is also possible.

[0162] Next, we examine another mode of system operation. FIG. 11 showsa graphical indication of two system flags: access point data status andCCA status. These flags may indicate, respectively, whether the accesspoint has LAN traffic to send to remote stations, and whether thechannel is clear. The fourth access point transmission in FIG. 11illustrates that, in the gap just preceding the transmission, the CCAindicates that the channel is clear and the access point indicates thatit has no data to send. If this is the case, the LPS digital signalprocessor and the MAC may then exchange handshake information across theRTS/CTS bus to hand control of system transmissions to the LPS digitalsignal processor. This change-of-command process may then initiate along PLCP PPDU that has a specific fixed data field and that usesstandard 802.11b signaling for the preamble and header, but that usesLPS-specific spreading sequences and spreading rates for the PSDU.

[0163] The PSDU length may be set by configuration data for this systemand may be under control of the administrator for the system. Theduration may be set to span from he minimum length packet allowed for802.11b to the maximum length packet. This may provide a level ofpriority control of the LPS portion of the system with respect to heWLAN part of the system. Short PSDUs may favor WLAN traffic over taglocation, while long PSDUs may favor LPS tag location over WLAN traffic.The length may vary with WLAN use.

[0164] Finally, for tag #34 in FIG. 11, the special LPS frame in 802.11bformat may be able to read at least one of the basic datagramtransmissions for this tag in low-resolution format (during the longPLCP header that lasts for 192 microseconds). Additionally, if the frameis long enough, other tags (such as tag #BC) may be read inhigh-resolution mode using the special PSDU part of the access pointtransmission.

[0165] Next, we examine the LPS baseband digital signal processor shownin FIG. 13. This may consist of two basic blocks: correlator banks andprocessing engines. Received baseband information may be sent to theappropriate correlator via a multiplexer depending on the currentconfiguration of the system (low-resolution short PLCP transmissions orhigh-resolution long PLCP transmissions). The correlator banks mayconsist of multiple XOR-based correlators that may bit-match and sum theincoming data with stored reference sequences. In the case of thelow-resolution correlator bank, the reference sequences may be the11-bit Barker sequence used in 802.11b preamble transmissions. In thecase of the high-resolution LPS transmissions, the reference sequencesmay be provided by the configuration setup to the PN sequence generatorand loader. In either case, the reference sequence may be shifted onebit later for each succeeding correlator in the correlator bank, thusproviding a RAKE-like correlator structure. Unlike in the RAKEcorrelator, however, the outputs of the individual correlators in thecorrelator banks may or may not be summed. Instead, the various enginesmay use the output data in various ways. The correlation-found enginemay scan the correlator output register and determine whether acorrelation signal exceeds the threshold set by the configuration. Ifso, it may signal the first echo detection engine to start and may passthe best tap (correlation peak location) to the data demodulationengine. The first echo detection and distance calculation engine maythen determine the time location of the first received echo, perhapsusing the RLS fitting procedure described in U.S. patent applicationSer. No. 09/244,600, filed Feb. 4, 1999.

[0166] Once the first echo detection engine has finished, it may enablethe data demodulation engine to begin parsing the output of thecorrelator bank's best tap, using the transmitted data as a reference.Both the data demodulation engine and the first echo detection enginemay provide outputs to the data table generation engine, which may sendthe result (i.e., tag ID coupled with distance from the access pointantenna) to a dual-port memory. One implementation of the system mayhave the dual-port memory sending data directly to the MAC unit thatprocesses returned data, forming the data into wired LAN Ethernetpackets. Another implementation may have the dual-port memory feeding anEthernet packet generator that may send the final data out over anEthernet physical interface, in parallel with the MAC Ethernet datapackets.

[0167] A lower-cost embodiment of the integrated system described abovemay omit a high-resolution correlation bank, wide-bandwidth LPS Txchain, and certain other features.

[0168] The two modes of the integrated system described above (i.e.,WLAN mode and is LPS mode) are illustrated in a different way in FIG.14. In access point mode, the access point may function as a standard802.11 transmit/receive radio. The transmit/receive radio may transmitsignals in the 2.4 GHz band to a nearby 802.11 device. Simultaneously,those transmitted signals may be received by tags within range of thetransmit/receive radio. A tag may convert these signals to the 5.8 GHzband and transmit the upconverted signals to the LPS receiver. The LPSreceiver may then process received signals to decode the tag's datagramand determine the distance between the access point/LPS and the tag. Inaddition, the LPS receiver may address the WLAN with a request totransmit data. If no 802.11 packets are being transmitted by the WLANaccess point's transmit/receive radio and the channel is clear, thesystem may switch to LPS mode. If the access point does not have data tosend, but the channel is not clear, the LPS transmitter may beconfigured to wait. If this situation persists, a timeout condition mayoccur, allowing the transmission to proceed regardless of WLAN on-airstatus. In LPS mode, the access point's transmit/receive radio maytransmit packets with the access point address. The packet sent from thetransmit/receive radio in this mode can begin as a valid 802.11 framewith source and destination fields the same (using the hardware addressof the access point). Since all other 802.11 terminals will ignore thepacket after address filtering, the frame's structure may then change tothe structure of a high-resolution LPS data frame, allowing the hybridsystem to exhibit better resolution in this mode than in access pointmode while preventing interference between LPS and WLAN.

[0169] Essentially, the system as described above allows the LPS to usebandwidth that is unused by the WLAN transmit/receive radio. Thus, theLPS may or may not interfere with WLAN transmission when the WLAN isusing the channel, depending, for example, on timeout parameters.However, when the WLAN is not transmitting, the LPS may use the unusedbandwidth for its own transmissions and provide better resolution thanpossible in simple access point mode. It is worth noting that theaforementioned integration may be accomplished in multiple ways. Amodification may be made to a WLAN access point to include LPScapabilities, configured (for example) as an integrated module or anoptional plug-in module. Similarly, an LPS interrogator may be modifiedto add the function of an access point's transmit/receive radio.

[0170] A tag may transpond 802.11 signals originating from a sourceother than the interrogator. Regardless of the original source of the802.11 signal, the interrogator's receiver section may decode theincoming data transponded from the tag at 5.8 GHz, as shown in FIG. 8.The “transmitter” section may operate instead as a normal 802.11receiver and may decode the incoming data arriving directly from thedata source at 2.44 GHz, thus providing a reference signal for decodingtag data. If the remote source of 802.11 signal is at a known location,it may be possible to determine the distance to the tag, as shown inFIG. 7. Interrogator A and Interrogator 13 may be at fixed positions,and the distance X between them may be pre-calibrated and known.Interrogator A may emit an 802.11 packet at 2.4 GHz, which the tag maytranspond to the 5.8 GHz band. This signal may be received byInterrogator A, and the distance Y between Interrogator A and the tagmay be calculated based on round trip time of flight. A 2.4 GHz signalmay be sent from Interrogator A to Interrogator B; this signal travelsacross known distance X. Interrogator B may also receive a signal fromthe tag at 5.8 GHz; this signal travels from Interrogator A to the tagand then to Interrogator B, and thus the signal travels across distance(Y+Z). The difference between the arrival times of the 2.4 GHz signaland the 5.8 GHz signal may be used to calculate ((Y+Z)−X). Since X and Yare known, simple substitution may be used to solve for Z. Thus, an802.11 packet from one interrogator may be used to measure the distancefrom the tag to several interrogators. Although this paragraph refers to“interrogators,” this is for economy of expression. Interrogator A maybe simultaneously acting as a WLAN device, with emissions addressed toanother 802.11 device other than Interrogator B. Similarly, InterrogatorB may be a dual-purpose WLAN/LPS device.

[0171] A high-performance version of the interrogator may use a 31 MHzchip rate and digitizes at 62 MHz, giving a “ruler” of 16.1 nanosecondsor about 8 feet (round trip), with resolution of approximately ±4 feetwithout interpolation. Interpolation techniques may improve thisresolution, as described in U.S. patent application Ser. No. 09/645,280,filed Aug. 24, 2000. An 802.11 radio uses an 11 MHz chip rate anddigitizes at 22 MHz, resulting in roughly one-third the resolution.Digitizing at a faster rate may provide improved interpolation. Sincemultiple reads are available, the effect of a faster digitization ratemay be achieved by operating the clocks of the transmit and receivedigital systems at slightly different rates, resulting in some variationin the sampling offset of the received signal. Another approachavailable in an 802.11 radio is to change the operating frequency of theinterrogator periodically to one of the eleven available centerfrequencies supported by 802.11, providing frequency diversity from onetag datagram to another.

[0172] Software Aspects of LPS and WLAN Integration

[0173] PinPoint has previously disclosed, for example in U.S. patentapplication Ser. No. 09/378,417, filed Aug. 20, 1999, apublish/subscribe software API for collecting data from interrogatorsand distributing this data across a network. Note that the nature of thedata from both the high-performance (e.g., 31 MHz) and lower-performance(e.g., 11 MHz) LPS embodiments is essentially identical, comprising TAD(tag-antenna-distance) readings from tags, with the only differencebeing the reliability and accuracy of the readings. Compatibilitybetween the two modes of operation is simply a matter of using the sameTCP/IP messages between the host and interrogator; all other software isthe same. For cost reasons, some features may be omitted from thelower-performance embodiment. Likewise, a combined 802.11 and LPS tagsensor may have an enhanced feature set. Such differences are reflectedin the configuration service on the host computer and the correspondingmessages sent between the host computer and the interrogator. Regardlessof such differences, the same fundamental information, originating fromthe same tags, is generated in both embodiments and the same real-timemessages and services may be used for both.

[0174] Integrating LPS and Other Wireless Technologies

[0175] Similar concepts to those described above may also be applied tocordless PBX products such as those operating in accordance with theDECT standard. Similar concepts may also be applied to systems operatingin accordance with other wireless in-building standards, for example,those operating in the 1.9 GHz band used for cordless PBX, or to otherwireless communication technologies, such as medical telemetry systems.

[0176] Similar concepts to those described above may be advantageouslyapplied to other radios. For example, these concepts may be applied tothe frequency-hopping version of 802.11 or to frequency-hoppingBluetooth radios. In these situations, some modifications may berequired. For example, frequency hoppers may use FSK it may beunnecessarily complicated to demodulate a BPSK tag signal. Therefore,On-Off Keying (OOK) or another form of amplitude modulation (AM) may beused by the tag, and demodulation may occur by simply noting thepresence or absence of energy in the expected sub-band around 5.8 GHz.

[0177] Integrating Other Systems with Non-Interfering Radio Frequencies

[0178] Other types of systems may also be integrated with WLANs andother in-building communication systems as described above, providedthat the systems incorporated in the combined infrastructure operate atnon-interfering frequencies. For example, Ultra High Frequency (UHF)beaconing RFID tag sensors may also be integrated with WLANs. UHFbeaconing tags, available from various vendors including Sovereign andRFCode, periodically transmit AM or FM modulated datagrams in UHF bandssuch as 303.8 MHz, 418 MHz, or 433 MHz. Readers for UHF beacon tags arerelatively simple in their design, and it may be advantageous tointegrate such systems with 802.11 WLANs or with DECT systems, forexample. As the frequencies used by these systems are typicallydifferent from those used for WLAN communication, integration maycomprise co-packaging a WLAN access point and a tag sensor, with minimalsharing of radio components, but with sharing of network connections,digital devices, packaging, marketing, and so forth.

[0179] Some potential applications of a system with integrated WLAN andRFID/LPS capabilities may be found in the home environment. For example,an individual may have a WLAN installed to network his or her homecomputers. If the WLAN access point has integrated RFID/LPScapabilities, the individual may attach RFID tags to the artwork andother valuables within his or her home and may use the trackingcapabilities of the system for a burglar alarm. A motion detector onsuch tags may be used to indicate tampering. Such a system may eliminatethe need to install a separate infrastructure for an alarm system. Otherhome applications may also be developed. For example, an RFID tag with atemperature sensor may notify the system when the refrigerator is notoperating correctly, or when the oven has reached a desired temperature.Numerous other uses for such integrated systems are possible both insideand outside of the home environment. These concepts may be applied notonly to 802.11 WLANs and beaconing RFID systems, but they may also beapplied to any similar systems that share operating frequencies or thatoperate using non-interfering frequencies as described above.

[0180] While the invention has been described with reference to variousillustrative embodiments, the invention is not limited to theembodiments described. Thus, it is evident that many alternatives,modifications, and variations of the embodiments described will beapparent to those skilled in the art. Accordingly, embodiments of theinvention as set forth herein are intended to be illustrative, notlimiting. Various changes may be made without departing from theinvention.

1. A method for controlling operations of a wireless communicationsystem and a wireless tag identification system having at leastpartially overlapping coverage areas, comprising: providing a wirelesscommunication system having at least two wireless communication devicesadapted to communicate by wireless signals; providing a wireless tagidentification system adapted to communicate by wireless signals with atleast one tag associated with an asset; and controlling the wirelesssignals produced by the wireless tag identification system to minimizeinterference of the wireless signals with wireless communication of thewireless communication system.
 2. The method of claim 1, wherein thestep of controlling the wireless signals comprises: adjusting a timingat which wireless signals are produced by the wireless tagidentification system.
 3. The method of claim 1, wherein the step ofcontrolling the wireless signals comprises: setting a duty cycle used tocontrol when wireless signals are permitted to be produced by thewireless tag identification system to minimize interference of thewireless signals with wireless communication of the wirelesscommunication system.
 4. The method of claim 1, wherein the step ofproviding a wireless communication system comprises: providing awireless local area network (WLAN).
 5. The method of claim 1, whereinthe step of providing a wireless tag identification system comprises:providing a wireless tag identification system that includes at leastone tag sensor that is physically separate from fixed communicationdevices in the wireless communication system.
 6. The method of claim 1,wherein the step of providing a wireless tag identification systemcomprises: providing a wireless tag identification system that uses afixed communication device in the wireless communication system tocommunicate with the tag.
 7. The method of claim 1, wherein the step ofproviding a wireless tag identification system comprises: providing awireless tag identification system that uses a fixed communicationdevice in the wireless communication system to send a wireless signal tothe tag, and that includes a tag sensor that receives a wireless signalfrom the tag.
 8. The method of claim 1, wherein the step of controllingthe wireless signals comprises: using a user selectable power todetermine the power of wireless signals sent by the wireless tagidentification system or a user selectable duty cycle to determine whenwireless signals are sent by the wireless tag identification system tothe tag.
 9. The method of claim 1, wherein the step of controlling thewireless signals comprises: synchronizing duty cycles of at least twointerrogators in the wireless tag identification system.
 10. The methodof claim 9, wherein the step of synchronizing duty cycles comprises:synchronizing the duty cycles of the at least two interrogators so thatthe interrogators are permitted to transmit wireless signals during acommon time period.
 11. The method of claim 9, wherein the step ofcontrolling the wireless signals further comprises: establishing a firsttag sensor as a master tag sensor and indicating to other tag sensorswhen the other tag sensors are permitted to transmit wireless signals.12. The method of claim 11, further comprising: receiving a schedule fora plurality of On/Off cycles to the other tag sensors.
 13. The method ofclaim 11, further comprising: sending a signal that indicates when theother tag sensors are permitted to transmit wireless signals at a timeat least equal to a transmission delay time in advance of a next periodduring which the other tag sensors are permitted to transmit wirelesssignals.
 14. The method of claim 1, wherein the step of controlling thewireless signals comprises: controlling a length of time during whichwireless signals are permitted to be produced by the wireless tagidentification system based on received signals from the at least onetag.
 15. The method of claim 1, wherein the step of controlling thewireless signals comprises: controlling a duty cycle for at least onetag sensor in the wireless tag identification system based on receivedsignals from the at least one tag.
 16. The method of claim 1, whereinthe step of controlling the wireless signals comprises: adjusting a timeperiod during which a wireless tag identification system is permitted totransmit wireless signals from a previous time period length toaccommodate an anticipated number of wireless signals to be receivedfrom tags.
 17. The method of claim 1, wherein the step of controllingthe wireless signals comprises: adjusting a duty cycle of at least onetag sensor in the wireless tag identification system from a first dutycycle setting to a second duty cycle setting based on previouslyreceived signals from tags to provide an adjusted percentage time duringwhich the wireless communication system is permitted to communicateusing wireless signals.
 18. The method of claim 1, wherein the step ofcontrolling the wireless signals comprises: adjusting a duty cycle or awireless signal power for at least one tag sensor in the wireless tagidentification system as a function of a time of day or a day of theweek.
 19. The method of claim 1, wherein the step of controlling thewireless signals comprises: adjusting a time period during which thewireless tag identification system is permitted to transmit wirelesssignals based on a received tag signal history.
 20. The method of claim1, wherein the step of controlling the wireless signals comprises:increasing a percentage time that the wireless tag identification systemis permitted to transmit wireless signals during lower communicationactivity periods for the wireless communication system; and decreasing apercentage time that the wireless tag identification system is permittedto transmit wireless signals during higher communication activityperiods for the wireless communication system.
 21. The method of claim1, wherein the step of controlling the wireless signals comprises:adjusting the timing at which wireless signals are produced by thewireless tag identification system to approximately coincide with a timewhen a tag is in an active state and enabled to send a signal, where thetag switches between an active state and a sleep state.
 22. The methodof claim 21, wherein the at least one tag switches between active andsleep states at a variable timing.
 23. The method of claim 21, furthercomprising: determining that the tag has not been identified by thewireless tag identification system for one of a time period and a numberof search cycles larger than a threshold; and the step of adjusting thetiming is performed in response to determining that the tag has not beenidentified.
 24. The method of claim 1, wherein the step of controllingthe wireless signals comprises: controlling a timing at which wirelesssignals are produced from a tag sensor in the wireless tagidentification system based on a location of the antenna.
 25. The methodof claim 24, wherein the step of controlling the wireless signalscomprises: controlling the timing for the tag sensor based on aproximity of the tag sensor to at least one fixed communication devicein the wireless communication system.
 26. The method of claim 25,wherein the step of controlling the wireless signals comprises:controlling the timing for the tag sensor based on a proximity of thetag sensor to at least one fixed communication device in the wirelesscommunication system and an amount of wireless communications traffichandled by the at least one fixed communication device.
 27. The methodof claim 24, wherein the step of controlling the wireless signalscomprises: setting a percentage time that tag sensors nearer a hightraffic area of the wireless communication system are permitted totransmit wireless signals to be lower than for tag sensors farther froma high traffic area of the wireless communication system.
 28. The methodof claim 1, wherein the step of controlling the wireless signalscomprises: determining if wireless signals related to the wirelesscommunications network are likely being transmitted; and permittingwireless signals to be produced by the wireless tag identificationsystem approximately while wireless signals related to the wirelesscommunication system are not being transmitted.
 29. The method of claim28, wherein the step of determining if wireless signals related to thewireless communications network are likely being transmitted comprises:detecting a change in energy in at least one communication channel. 30.The method of claim 28, wherein the step of determining if wirelesssignals related to the wireless communications network are likely beingtransmitted comprises: adjusting a length of a time during which anabsence of wireless signal energy is detected.
 31. The method of claim30, wherein the step of determining if wireless signals related to thewireless communications network are likely being transmitted comprises:randomly adjusting the length of time during which the absence ofwireless signal energy is detected.
 32. The method of claim 1, whereinthe step of controlling the wireless signals comprises: determining ifenergy in a communication channel that is indicative of wireless signalsrelated to the wireless communications network is absent for a periodlonger than a threshold; and permitting wireless signals to be producedby the wireless tag identification system.
 33. The method of claim 28,further comprising: adjusting a process used to determine if thewireless signals are likely being transmitted so that the wireless tagidentification system is not prevented from transmitting wirelesssignals for more than a threshold percentage of time.
 34. The method ofclaim 28, further comprising: adjusting a process used to determine ifthe wireless signals are likely being transmitted so that the wirelesstag identification system is prevented from transmitting wirelesssignals for a target percentage of time.
 35. The method of claim 28,wherein the step of determining is wireless signals related to thewireless communications network are likely being transmitted comprises:detecting energy, that is indicative of wireless signals related to thewireless communications network, near a tag sensor that is part of thewireless tag identification system.
 36. The method of claim 28, whereinthe step of determining if wireless signals related to the wirelesscommunications network are likely being transmitted comprises: detectingenergy, that is indicative of wireless signals related to the wirelesscommunications network, near a high traffic area of the wirelesscommunication system.
 37. The method of claim 28, wherein the step ofdetermining if wireless signals related to the wireless communicationsnetwork are likely being transmitted comprises: determining a pluralityof statuses that are each indicative of the presence of wireless signalsrelated to the wireless communications network; and using an average ofthe statuses to control the timing at which wireless signals arepermitted to be produced by the wireless tag identification system. 38.The method of claim 37, wherein the step of determining a plurality ofstatuses comprises: detecting energy that is indicative of wirelesssignals related to the wireless communications network; and inferringthe status of the wireless communications network for each energydetection occurrence.
 39. The method of claim 38, wherein the step ofdetermining the status comprises: comparing a signal representative ofthe detected energy to one of a fixed threshold and a variablethreshold.
 40. The method of claim 28, wherein the step of determiningif wireless signals related to the wireless communications network arelikely being transmitted comprises: detecting energy that is indicativeof wireless signals related to the wireless communications network in aplurality of channels used by the wireless communication system.
 41. Themethod of claim 28, wherein the step of determining if wireless signalsrelated to the wireless communications network are likely beingtransmitted comprises: detecting energy that is indicative of wirelesssignals related to the wireless communications network in a channel usedby the wireless communication system that is not a channel used by thewireless tag identification system.
 42. The method of claim 1, furthercomprising: sending a signal between the wireless tag identificationsystem and the wireless communication system that indicates a control ofthe timing at which wireless signals are permitted to be produced by thewireless tag identification system.
 43. The method of claim 42, whereinthe step of sending a signal comprises: sending a signal from thewireless communication system to the wireless tag identification systemthat indicates when the wireless tag identification system is permittedto transmit wireless signals.
 44. The method of claim 43, furthercomprising: using the signal to control a duty cycle for at least onetag sensor of the wireless tag identification system.
 45. The method ofclaim 42, wherein the step of sending a signal comprises: sending atoken between the wireless tag identification system and the wirelesscommunication system, where a system holding the token controls use ofat least one wireless communication channel.
 46. The method of claim 42,further comprising: providing a host system adapted to communicate withthe wireless tag identification system and the wireless communicationsystem; and the step of sending a signal comprises sending a signal fromthe host system to at least one of the wireless tag identificationsystem and the wireless communication system representative of whetherthe wireless tag identification system or the wireless communicationsystem may use a wireless communication channel.
 47. The method of claim42, wherein the step of sending a signal comprises: sending the signalvia a physical medium.
 48. The method of claim 42, wherein one of thewireless tag identification system and the wireless communication systemis a master system, and the other of the wireless tag identificationsystem and the wireless communication system is a slave system, andwherein the master system controls which system may use one or morecommunication channels.
 49. The method of claim 48, wherein the wirelesscommunication system is the master system and controls when the wirelesstag identification system is permitted to use at least onecommunications channel.
 50. The method of claim 41, wherein the step ofsending a signal comprises: sending signals both from the wireless tagidentification system to the wireless communication system and from thewireless communication system to the wireless tag identification systemto control the timing at which wireless signals are permitted to beproduced by the wireless tag identification system.
 51. A method foridentifying tags comprising: providing at least one tag adapted totransmit a wireless signal; providing a wireless tag identificationsystem adapted to receive a wireless signal from the at least one tagand identify the tag; using a first technique to determine thelikelihood that the tag is within acceptable communication range; andusing a second technique to collect data from the tag if the tag isdetermined likely to be within an acceptable communication range. 52.The method of claim 51, wherein the step of using a first techniquecomprises: adjusting a tag search procedure based on at least one signalreceived from tags.
 53. The method of claim 51, wherein the step ofusing a first technique comprises: measuring whether an energy of areceived signal is not greater than a threshold.
 54. The method of claim51, wherein the step of using a first technique comprises: aborting afull tag search procedure that includes a plurality of sequences if acorrelated magnitude of a signal received in a first sequence is below athreshold.
 55. The method of claim 54, wherein each sequence is a127-chip sequence.
 56. The method of claim 54, wherein the firstsequence includes 31-chip sequences, and other sequences are 127-chipsequences.
 57. The method of claim 54, wherein the first sequenceincludes a plurality of first chip sequences and the other sequences inthe search procedure include a second chip sequence, where the firstchip sequence is shorter than the second chip sequence.
 58. The methodof claim 51, wherein the step of using a first technique comprises:using a subset of tag sensors in the wireless tag identification systemto identify the presence of a tag; and the step of using a secondtechnique comprises: using at least one tag sensor not in the subset tocollect data from the tag.
 59. The method of claim 51, wherein the stepof using a first technique comprises: using a first set of wirelesssignal frequency bands to identify the presence of a tag; and the stepof using a second technique comprises: using a second set of wirelesssignal frequency bands to collect data from the tag.
 60. The method ofclaim 51, wherein the step of using a first technique comprises: using afrequency hopper radio to identify the presence of a tag.
 61. The methodof claim 51, wherein the step of using a first technique comprises:using one type of signal used to search for tags; and the step of usingthe second technique comprises: using another type of signal to collectdata from the tag.
 62. The method of claim 61, wherein the step of usingthe second technique comprises: estimating a location of the tag inrelation to one or more tag sensors.
 63. The method of claim 61, whereinthe step of using the second technique comprises: reading bits of datafrom the tag.
 64. The method of claim 61, wherein the step of using onetype of signal comprises: using a signal in a narrow band; and the stepof using another type of signal comprises: using a signal in a band thatis wider than the narrow band.
 65. The method of claim 61, wherein thestep of using one type of signal comprises: using a frequency hoppingsignal; and the step of using another type of signal comprises: using adirect sequence spread spectrum signal.
 66. The method of claim 61,wherein the step of using one type of signal comprises: using a directsequence spread spectrum signal at a first chip rate; and the step ofusing another type of signal comprises: using a direct sequence spreadspectrum signal at a second chip rate higher than the first chip rate.67. The method of claim 61, wherein the step of using one type of signalcomprises: using a signal including a communications network packet; andthe step of using another type of signal comprises: using a signalincluding a special purpose packet different from the network packet.68. The method of claim 61, wherein the step of using one type of signalcomprises: using a sequence spread spectrum signal having a firstlength; and the step of using another type of signal comprises: using asequence spread spectrum signal having a second length longer than thefirst length.
 69. The method of claim 68, wherein the first length isone of 11, 15 and 31 chips and the second length is one of 31, 63 and127 chips.
 70. The method of claim 61, wherein the step of using onetype of signal comprises: using a single signal; and the step of usinganother type of signal comprises: using multiple signals.
 71. The methodof claim 61, wherein the step of using one type of signal comprises:using a subset of tag sensors in the wireless tag identification systemwith the one type of signal to identify the presence of a tag; and thestep of using a second technique comprises: selecting tag sensors tocollect data from the tag depending on the tag that is identified. 72.The method of claim 51, wherein the step of using a first techniquecomprises: determining if a signal received from a tag is above athreshold; and the step of using second technique is performed if thesignal is above the threshold.
 73. The method of claim 51, wherein thestep of using a first technique comprises: using a signal having a firstpower level; and the step of using the second technique comprises: usinga signal having a second power level different from the first powerlevel.
 74. A method for identifying assets comprising: providing atleast one tag adapted to transmit a wireless signal; providing awireless tag identification system adapted to receive a wireless signalfrom the at least one tag and determine a location for the tag; andreceiving a wireless signal from the tag including a tag datagram inwhich an error checking code portion of the tag datagram is transmittedat the start of the tag datagram.
 75. A method for identifying assetscomprising: providing a plurality of tags adapted to transmit wirelesssignals including different length header portions; and providing awireless tag identification system adapted to receive a wireless signalfrom the tags and determine a location for the tags.
 76. The method ofclaim 75, wherein the step of providing a plurality of tags comprises:providing first and second tags adapted to transmit a wireless signalincluding a header portion, a first tag header portion of the first tagbeing longer than a second tag header portion of the second tag.
 77. Themethod of claim 75, wherein the step of providing a plurality of tagscomprises: providing a tag adapted to transmit a wireless signalselectively including either a first header portion or a second headerportion, a first tag header portion of the tag being longer than asecond tag header portion of the tag; and providing a wireless tagidentification system adapted to receive a wireless signal from the atleast one tag and determine an identity of the tag.
 78. The method ofclaim 77, further comprising: transmitting the wireless signal includingthe first header portion when the tag is in motion; and transmitting thewireless signal including the second header portion when the tag isstationary.
 79. The method of claim 75, wherein the step of providing aplurality of tags comprises: providing tags adapted to transmit awireless signal including a longest header portion, the longest headerportion having a transmission time that is at least as long as a tagsearch cycle time for tag search procedures performed by the wirelesstag identification system to identify the presence of tags.
 80. Themethod of claim 79, wherein the tag search cycle time is a time measuredfrom the start of a first tag search procedure to the start of a nexttag search procedure.
 81. The method of claim 80, wherein the longestheader portion transmission time is approximately equal to the tagsearch cycle time.
 82. The method of claim 76, further comprising:operating the first tag in a detection mode in which the first tagdetects whether received wireless signal energy is above a threshold,and in a transmit mode in which the first tag performs a transmissionprocess if the received wireless signal energy is above the threshold.83. The method of claim 82, further comprising: operating the tag toperform the transmission process for some proportion of the header evenif the received wireless signal energy is below the threshold.
 84. Amethod for communicating with communication devices in a wirelesscommunication system and tags associated with assets in a wireless tagidentification system, comprising: sending and receiving wirelesssignals to and from communication devices in the wireless communicationsystem; receiving a second wireless signal sent from a tag in responseto a first wireless signal, said first wireless signal being sent fromat least one communication device in the wireless communication system,and said first wireless signal not being addressed to the tag; and usingthe second wireless signal to estimate the location of an asset.
 85. Themethod of claim 84, wherein the first wireless signal is sent from afixed communication device that has a destination address of the fixedcommunication device.
 86. The method of claim 84, wherein the firstwireless signal is modified to be different from a standard WLAN signal.87. The method of claim 86, wherein the first wireless signal ismodified to increase a sequence spreading rate of the signal.
 88. Amethod for determining a location for assets, comprising: providing aplurality of assets; producing a wireless communication signal involvinga mobile device, the wireless communication signal representingcommunications audio, video or data information; using a frequencyshifting transponder in conjunction with the wireless communicationsignal to locate at least one of the assets.
 89. The method of claim 88,wherein the wireless communication signal is an 802.11 compliant signal.90. The method of claim 88, wherein the frequency shifting transpondershifts a frequency of the wireless communication signal fromapproximately 2.4 GHz to 5.8 GHz.
 91. The method of claim 88, whereinthe wireless communication signal includes at least one of a lowresolution packet that is compliant with a wireless communicationstandard and a high resolution packet that is not compliant with thewireless communication standard.
 92. The method of claim 91, wherein thehigh resolution packet includes a header that is compliant with thewireless communication standard.
 93. The method of claim 92, wherein thehigh resolution packet includes a compliant header portion and anon-compliant data portion.
 94. The method of claim 91, wherein thewireless communication signal includes both a low resolution packet anda high resolution packet.
 95. The method of claim 91, wherein aplurality of wireless communication signals are produced and the highresolution packets are more frequent or longer when wirelesscommunication signals are less frequently produced.
 96. The method ofclaim 91, wherein a plurality of wireless communication signals areproduced and the high resolution packets are more frequent or longerwhen locating assets has a higher priority in comparison to wirelesscommunication involving mobile devices.
 97. A wireless tagidentification system, comprising: a plurality of tags each associatedwith an asset; at least one tag sensor adapted to communicate bywireless signals with at least one tag, the at least one tag sensorhaving a coverage area within which the tag sensor can communicate withtags; and means for controlling wireless signals produced by the atleast one tag sensor to minimize interference of the wireless signalswith communication of a wireless communication system taking placewithin the coverage area of the at least one tag sensor.
 98. The systemof claim 97, wherein the means for controlling the wireless signalsadjusts a timing at which wireless signals are produced by the wirelesstag identification system.
 99. The system of claim 97, wherein the meansfor controlling the wireless signals adjusts a power level of thewireless signals.
 100. The system of claim 97, wherein the means forcontrolling the wireless signals sets a duty cycle used to control whenwireless signals are permitted to be produced by the wireless tagidentification system to minimize interference of the wireless signalswith wireless communication of the wireless communication system. 101.The system of claim 97, wherein the wireless communication systemcomprises a wireless local area network (WLAN).
 102. Thc system of claim97, wherein the wireless tag identification system comprises at leastone tag sensor that is physically separate from fixed communicationdevices in the wireless communication system.
 103. The system of claim97, wherein the wireless tag identification system is adapted to use afixed communication device in the wireless communication system tocommunicate with the tag.
 104. The system of claim 97, furthercomprising: means for allowing a user to select a power of wirelesssignals sent by the wireless tag identification system or to select aduty cycle to determine when wireless signals are sent by the wirelesstag identification system to the tag.
 105. The system of claim 97,wherein the means for controlling the wireless signals synchronizes dutycycles of at least two tag sensors in the wireless tag identificationsystem.
 106. The system of claim 105, wherein a first tag sensor is amaster tag sensor and other tag sensors are notified when the other tagsensors are permitted to transmit wireless signals.
 107. The system ofclaim 106, wherein a schedule for a plurality of On/Off cycles is sentto the other tag sensors.
 108. The system of claim 106, wherein a signalthat indicates when the other tag sensors are permitted to transmitwireless signals is sent at a time at least equal to a transmissiondelay time in advance of a next period during which the other tagsensors are permitted to transmit wireless signals.
 109. The system ofclaim 97, wherein the means for controlling controls a length of timeduring which wireless signals are permitted to be produced by thewireless tag identification system based on received signals from the atleast one tag.
 110. The system of claim 97, wherein the means forcontrolling controls a duty cycle for at least one tag sensor in thewireless tag identification system based on received signals from the atleast one tag.
 111. The system of claim 97, wherein the means forcontrolling adjusts a time period during which a wireless tagidentification system is permitted to transmit wireless signals from aprevious time period length to accommodate an anticipated number ofwireless signals to be received from tags.
 112. The system of claim 97,wherein the means for controlling adjusts a duty cycle of at least onetag sensor in the wireless tag identification system from a first dutycycle setting to a second duty cycle setting based on previouslyreceived signals from tags to provide an adjusted percentage time duringwhich the wireless communication system is permitted to communicateusing wireless signals.
 113. The system of claim 97, wherein the meansfor controlling adjusts a duty cycle or a wireless signal power for atleast one tag sensor in the wireless tag identification system as afunction of a time of day or a day of the week.
 114. The system of claim97, wherein the means for controlling adjusts a time period during whichthe a tag sensor is permitted to transmit wireless signals based on areceived tag signal history.
 115. The system of claim 97, wherein themeans for controlling increases a percentage time that a tag sensor ispermitted to transmit wireless signals during lower communicationactivity periods for the wireless communication system; and decreases apercentage time that the wireless tag identification system is permittedto transmit wireless signals during higher communication activityperiods for the wireless communication system.
 116. The system of claim97, wherein the means for controlling adjusts the timing at whichwireless signals are produced by a tag sensor to approximately coincidewith a time when a tag is in an active state and enabled to send asignal, where the tag switches between an active state and a sleepstate.
 117. The system of claim 116, wherein the tag switches betweenactive and sleep states at a variable timing.
 118. The system of claim116, wherein the means for controlling determines that the tag has notbeen identified by the wireless tag identification system for one of atime period and a number of search cycles larger than a threshold; andadjusts the timing in response to determining that the tag has not beenidentified.
 119. The system of claim 97, wherein the means forcontrolling controls a timing at which wireless signals are producedfrom a tag sensor in the wireless tag identification system based on alocation of an antenna associated with the tag sensor.
 120. The systemof claim 119, wherein the means for controlling controls the timing forthe tag sensor based on a proximity of a tag sensor antenna to at leastone fixed communication device in the wireless communication system.121. The system of claim 120, wherein the means for controlling controlsthe timing for the tag sensor based on a proximity of a tag sensorantenna to at least one fixed communication device in the wirelesscommunication system and an amount of wireless communications traffichandled by the at least one fixed communication device.
 122. The systemof claim 119, wherein the means for controlling sets a percentage timethat tag sensors nearer a high traffic area of the wirelesscommunication system are permitted to transmit wireless signals to belower than for tag sensors farther from a high traffic area of thewireless communication system.
 123. The system of claim 97, wherein themeans for controlling determines if wireless signals related to thewireless communications network are likely being transmitted; andpermits wireless signals to be produced by a tag sensor approximatelywhile wireless signals related to the wireless communication system arenot being transmitted.
 124. The system of claim 123, wherein the meansfor controlling detects a change in energy in at least one communicationchannel.
 125. The system of claim 123, wherein the means for controllingadjusts a length of a time during which an absence of wireless signalenergy is detected.
 126. The system of claim 125, wherein the means forcontrolling randomly adjusts the length of time during which the absenceof wireless signal energy is detected.
 127. The system of claim 97,wherein the means for controlling determines if energy in acommunication channel that is indicative of wireless signals related tothe wireless communications network is absent for a period longer than athreshold; and permits wireless signals to be produced by a tag sensor.128. The system of claim 123, wherein the means for controlling adjustsa process used to determine if the wireless signals are likely beingtransmitted so that the wireless tag identification system is notprevented from transmitting wireless signals for more than a thresholdpercentage of time.
 129. The system of claim 123, wherein the means forcontrolling adjusts a process used to determine if the wireless signalsare likely being transmitted so that the wireless tag identificationsystem is prevented from transmitting wireless signals for a targetpercentage of time.
 130. The system of claim 123, wherein the means forcontrolling detects energy, that is indicative of wireless signalsrelated to the wireless communications network, near a tag sensor thatis part of the wireless tag identification system.
 131. The system ofclaim 123, wherein the means for controlling detects energy, that isindicative of wireless signals related to the wireless communicationsnetwork, near a high traffic area of the wireless communication system.132. The system of claim 123, wherein the means for controllingdetermines a plurality of statuses that are each indicative of thepresence of wireless signals related to the wireless communicationsnetwork; and uses an average of the statuses to control the timing atwhich wireless signals are permitted to be produced by the tag sensor.133. The system of claim 132, wherein the means for controlling detectsenergy that is indicative of wireless signals related to the wirelesscommunications network; and infers the status of the wirelesscommunications network for each energy detection occurrence.
 134. Thesystem of claim 133, wherein the means for controlling compares a signalrepresentative of the detected energy to one of a fixed threshold and avariable threshold.
 135. The system of claim 123, wherein the means forcontrolling detects energy that is indicative of wireless signalsrelated to the wireless communications network in a plurality ofchannels used by the wireless communication system.
 136. The system ofclaim 123, wherein the means for controlling detects energy that isindicative of wireless signals related to the wireless communicationsnetwork in a channel used by the wireless communication system that isnot a channel used by the wireless tag identification system.
 137. Thesystem of claim 97, further comprising: a communication link thatcarries a signal between the wireless tag identification system and thewireless communication system that indicates a control of the timing atwhich wireless signals are permitted to be produced by the wireless tagidentification system.
 138. The system of claim 137, wherein the signalis sent from the wireless communication system to the wireless tagidentification system that indicates when the wireless tagidentification system is permitted to transmit wireless signals. 139.The system of claim 138, wherein the means for controlling uses thesignal to control a duty cycle for at least one tag sensor of thewireless tag identification system.
 140. The system of claim 137,wherein the signal includes a token sent between the wireless tagidentification system and the wireless communication system, where asystem holding the token controls use of at least one wirelesscommunication channel.
 141. The system of claim 137, further comprising:a host system adapted to communicate with the wireless tagidentification system and the wireless communication system, and adaptedto send a signal to at least one of the wireless tag identificationsystem and the wireless communication system representative of whetherthe wireless tag identification system or the wireless communicationsystem may use a wireless communication channel.
 142. The system ofclaim 137, wherein one of the wireless tag identification system and thewireless communication system is a master system, and the other of thewireless tag identification system and the wireless communication systemis a slave system, and wherein the master system controls which systemmay use one or more communication channels.
 143. The system of claim142, wherein the wireless communication system is the master system andcontrols when the wireless tag identification system is permitted to useat least one communications channel.
 144. The system of claim 97,wherein a clock in at least one tag sensor is operated at a differentrate than at least one other tag sensor, resulting in a variation in asampling offset of a signal received from at least one tag.
 145. Thesystem of claim 97, wherein an operating frequency of at least one tagsensor is adjusted periodically to provide frequency diversity from onesignal received from a tag to another signal received from a tag.
 146. Awireless tag identification system for identifying tags comprising: atleast one tag adapted to produce a wireless signal; at least one tagsensor that receives a wireless signal from the at least one tag; meansfor determining an identity of the tag based on the wireless signalreceived from the tag; and means for controlling how wireless signalsare generated by the tag sensor, the means for controlling using a firsttechnique to determine the likelihood that the tag is within acceptablecommunication range, and using a second technique to collect data fromthe tag if the tag is determined likely to be within an acceptablecommunication range.
 147. The system of claim 146, wherein the firsttechnique comprises: adjusting a tag search procedure based on at leastone signal received from tags.
 148. The system of claim 146, wherein thestep of using a first technique comprises: measuring whether an energyof a received signal is not greater than a threshold.
 149. The system ofclaim 146, wherein the step of using a first technique comprises:aborting a full tag search procedure that includes a plurality ofsequences if a correlated magnitude of a signal received in a firstsequence is below a threshold.
 150. The system of claim 149, whereineach sequence is a 127-chip sequence.
 151. The system of claim 149,wherein the first sequence includes 31-chip sequences, and othersequences are 127-chip sequences.
 152. The system of claim 149, whereinthe first sequence includes a plurality of first chip sequences and theother sequences in the search procedure include a second chip sequence,where the first chip sequence is shorter than the second chip sequence.153. The system of claim 146, wherein the first technique comprises:using a subset of tag sensors to identify the presence of a tag; and thestep of using a second technique comprises: using at least one tagsensor not in the subset to collect data from the tag.
 154. The systemof claim 146, wherein the first technique comprises: using a first setof wireless signal frequency bands to identify the presence of a tag;and the step of using a second technique comprises: using a second setof wireless signal frequency bands to collect data from the tag. 155.The system of claim 146, wherein the first technique comprises: using afrequency hopper radio to identify the presence of a tag.
 156. Thesystem of claim 146, wherein the first technique comprises: using afirst type of signal used to search for tags; and the step of using thesecond technique comprises: using a second type of signal to collectdata from the tag.
 157. The system of claim 156, wherein the secondtechnique comprises: estimating a location of the tag in relation to oneor more tag sensors.
 158. The system of claim 156, wherein the secondtechnique comprises: reading bits of data from the tag.
 159. The systemof claim 156, wherein the first type of signal comprises a signal in anarrow band; and the second type of signal comprises a signal in a bandthat is wider than the narrow band.
 160. The system of claim 156,wherein the first type of signal comprises a frequency hopping signal;and the second type of signal comprises a direct sequence spreadspectrum signal.
 161. The system of claim 156, wherein the first type ofsignal comprises a direct sequence spread spectrum signal at a firstchip rate; and the second type of signal comprises a direct sequencespread spectrum signal at a second chip rate higher than the first chiprate.
 162. The system of claim 156, wherein the first type of signalcomprises a signal including a communications network packet; and thesecond type of signal comprises a signal including special purposepacket different from the network packet.
 163. The system of claim 156,wherein the first type of signal comprises a sequence spread spectrumsignal having a first length; and the second type of signal comprises asequence spread spectrum signal having a second length longer than thefirst length.
 164. The system of claim 163, wherein the first length isone of 11, 15 and 31 chips and the second length is one of 31, 63 and127 chips.
 165. The system of claim 156, wherein the first type ofsignal comprises a single signal; and the second type of signalcomprises multiple signals.
 166. The system of claim 156, wherein thefirst technique comprises: using a subset of tag sensors in the wirelesstag identification system with a first type of signal to identify thepresence of a tag; and the second technique comprises: selecting tagsensors to collect data from the tag depending on the tag that isidentified.
 167. The system of claim 146, wherein the first techniquecomprises: determining if a signal received from a tag is above athreshold; and the second technique is performed if the signal is abovethe threshold.
 168. A system for identifying assets comprising: at leastone tag adapted to produce a wireless signal including a tag datagram inwhich an error checking code portion of the tag datagram is transmittedat the start of the tag datagram; at least one tag sensor adapted toreceive a wireless signal from the at least one tag; and means fordetermining a location for the tag based on the received wirelesssignal.
 169. A system for identifying assets comprising: a plurality oftags adapted to produce wireless signals including different lengthheader portions; at least one tag sensor adapted to receive a wirelesssignal from the at least one tag; and means for determining a locationfor the tag based on the received wireless signal.
 170. The system ofclaim 169, wherein first and second tags are adapted to transmit awireless signal including a header portion, a first tag header portionof the first tag being longer than a second tag header portion of thesecond tag.
 171. The system of claim 169, wherein at least one tag isadapted to transmit a wireless signal selectively including either afirst header portion or a second header portion, a first tag headerportion of the tag being longer than a second tag header portion of thetag.
 172. The system of claim 171, wherein at least one tag is adaptedto transmit the wireless signal including the first header portion whenthe tag is in motion, and to transmit the wireless signal including thesecond header portion when the tag is stationary.
 173. The system ofclaim 169, wherein a longest header portion transmitted by at least oneof the tags has a transmission time that is at least as long as a tagsearch cycle time for tag search procedures performed to identify thepresence of tags.
 174. The system of claim 173, wherein the tag searchcycle time is a time measured from the start of a first tag searchprocedure to the start of a next tag search procedure.
 175. The systemof claim 174, wherein the longest header portion transmission time isapproximately equal to the tag search cycle time.
 176. The system ofclaim 169, wherein at least one first tag is adapted to operate in adetection mode in which the first tag detects whether received wirelesssignal energy is above a threshold, and in a transmit mode in which thefirst tag performs a transmission process if the received wirelesssignal energy is above the threshold.
 177. The system of claim 176,wherein the first tag is adapted to perform the transmission process forsome proportion of the header even if the received wireless signalenergy is below the threshold.
 178. An integrated system forcommunicating with communication devices in a wireless communicationsystem and tags associated with assets in a wireless tag identificationsystem, comprising: means for sending and receiving wireless signals toand from communication devices in the wireless communication system;means for receiving a second wireless signal sent from a tag in responseto a first wireless signal, said first wireless signal being sent fromat least one communication device in the wireless communication system,and said first wireless signal not being addressed to the tag; and meansfor using the second wireless signal to estimate the location of anasset.
 179. The system of claim 178, wherein the first wireless signalis sent from a fixed communication device in the wireless communicationsystem that has a destination address of the fixed communication device.180. The system of claim 178, wherein the first wireless signal ismodified to be different from a standard WLAN signal.
 181. The system ofclaim 180, wherein the first wireless signal is modified to increase asequence spreading rate of the signal.
 182. A system for determining alocation for assets, comprising: means for producing a wirelesscommunication signal in a wireless communication system including amobile communication device, the wireless communication signalrepresenting communications audio, video or data information; and assetlocating means, including at least one frequency shifting transponder,for using the wireless communication signal to locate at least one ofthe assets.
 183. The system of claim 182, wherein the wirelesscommunication signal is an 802.11 compliant signal.
 184. The system ofclaim 182, wherein the frequency shifting transponder shifts a frequencyof the wireless communication signal from approximately 2.4 GHz to 5.8GHz.
 185. The system of claim 182, wherein the wireless communicationsignal includes at least one of a low resolution packet that iscompliant with a wireless communication standard and a high resolutionpacket that is not compliant with the wireless communication standard.186. The system of claim 185, wherein the high resolution packetincludes a header that is compliant with the wireless communicationstandard.
 187. The system of claim 186, wherein the high resolutionpacket includes a compliant header portion and a non-compliant dataportion.
 188. The system of claim 185, wherein the wirelesscommunication signal includes both a low resolution packet and a highresolution packet.
 189. The system of claim 185, wherein a plurality ofwireless communication signals are produced and the high resolutionpackets are more frequent or longer when wireless communication signalsare less frequently produced.
 190. The system of claim 185, wherein aplurality of wireless communication signals are produced and the highresolution packets are more frequent or longer when locating assets hasa higher priority in comparison to wireless communication involvingmobile devices.